METHANE PRODUCTION SYSTEM OF METHANE TANK TECHNICAL FIELD The present invention belongs to methane technique fields, and relates to a methane production system, more particularly, to a methane production system of a methane tank. BACKGROUND Along with economy development, people overuse non-renewable resources, which makes that each country faces severe tests on energy problems. The energy becomes an important security for sustainable development of a country economy. Methane is a clean energy in our country villages. It is produced by fermentation of abandoned straw of crops. It can be used for boiling water, cooking rice, etc. and can save a lot of natural resources, such as coals, woods, etc. It does not cause pollution to the environment after the methane fires and heat efficiency of the methane is high. At present, methane yield of a methane tank is generally limited. For example, methane yield is influenced by temperature change of seasons and day and night. Generally, when a temperature inside the methane tank is bigger than 150, methane is produced normally. When the temperature inside the methane tank is smaller than 150, production efficiency of methane is low, or even no methane is produced. Furthermore, people use a gas container or a gas storage bag to collect methane produced by the methane tank. This causes inconvenience of collecting gas, and space occupied by the gas -1 container is large. There are a lot of the above shortcomings that exist in the prior production system of methane. Therefore, for those skilled in the art, a new methane production system is to be developed to solve the shortcomings, such as low production efficiency of methane, inconvenience of collecting gas that exist in the prior methane tank. Therefore, more clean energy is provided for human being and sustainable development of economy can be moved forwards. SUMMARY OF THE INVENTION It is an object of the present invention is to provide a methane production system of a methane tank against the shortcomings that exist in the prior technique. The methane production system has features of high gas production efficiency, convenience of collecting gas and small space occupied by a storage device. The object of the present invention is achieved by the following technique solution: a methane production system of a methane tank includes a methane tank. The methane tank includes a tank body that can produce methane, a cover and a connecting pipe pivotedly connected to an upper part of the tank body. An electric heating pipe is set in the tank body. An end of the connecting pipe extends through the cover and is in communication with the tank body. The connecting pipe and the cover keep airtight. Other end of the connecting pipe is connected to a storage device. A control mechanism that automatically turns on the electric heating pipe when a temperature in the tank body is smaller than a set value is set in the tank body. The methane production system of the methane tank turns on the -2electric heating pipe by the control mechanism to heat the methane tank when the temperature in the methane tank is low, thereby increasing production efficiency of the methane tank. The methane produced by the methane tank can be collected quickly by the storage device. In the methane production system of the methane tank, the storage device includes a base and a number of gas storage bags having gas storage chambers. A post extends from the base along a vertical axis. A mid portion of the gas storage bag defines an installation hole. The gas storage chamber and the installation hole keep airtight. An air pipe is hermetically fixed on the gas storage bag and is in communication with the gas storage chamber. The gas storage bags are successively sleeved on the post. The air pipes connect the gas storage chambers of the gas storage bags in series using a pipe structure. A pressing mechanism is sleeved on the post and is used for pressing the gas storage bags on the base. The gas storage bag includes a ring-shaped first bag body and a ring-shaped second bag body. The first bag body and the second bag body are attached firmly to each other, and inner and outer edges of the first bag body are hermetically connected to inner and outer edges of the second bag body. A mid ring-shaped hole of the first bag body and a mid ring-shaped hole of the second bag body cooperatively form the installation hole. A chamber between the first bag body and the second bag body forms the gas storage chamber. An end of the air pipe is hermetically fixed to the first bag body or the second bag body and the air pipe is in communication with the gas storage chamber. Another end of the air pipe is a closed end. A pipe body of the air pipe defines a connecting hole in communication with an internal channel of the air pipe. The pipe structure includes a number of connecting valves and a main pipe. -3- An end of the main pipe has a communicating port. The communicating port is in communication with the connecting pipe. Another end of the main pipe is a closed end. An end of the connecting valve is hermetically connected to the connecting hole of the air pipe. Another end of the connecting valve is hermetically fixed to a pipe body of the main pipe and is in communication with an internal channel of the main pipe. Following is the work principle of the storage device. The gas storage bags successively sleeved on the post makes the methane in the methane tank transmit in the main pipe through the connecting pipe. The pipe structure can be collected and stored by flowing into the gas storage bag through the pipe structure and the air pipe, which is convenient. The gas storage bag on the post 6 can be pressed towards the base by the pressing mechanism, which minimizes volume of the gas storage bags, thereby reducing space occupied by the gas storage bags. Specifically, the gas storage bag is made plastic material. This is convenient for manufacture and cost is low. Further, tightness is good and it is easy to carry. The end of the air pipe is connected to the gas storage bag, and another end of the air pipe is a closed end and the methane can be imported from the side of the air pipe through the connecting valve. This makes stack of the gas storage bags smoother. The connecting valve is used for connecting and disconnecting the main pipe with the gas storage bag, which achieves automatic control of transmitting methane. In the methane production system of the methane tank, a bearing housing is fixed on the base. The bearing housing has a bearing inside. The post includes a lower portion and an upper portion fixed on the lower portion. The lower portion extends through and fixed on an inner ring of the bearing. The upper portion has external _4_ threads. This design makes the post rotate with the inner ring of the bearing on the base. In the methane production system of the methane tank, the pressing mechanism includes a pressing plate, a spring and a nut. A mid portion of the pressing plate defines a lifting hole. The post extends through the lifting hole. The nut is threadedly connected on the upper portion of the post and the spring is in a compressed state. An upper end of the spring is fixed on the nut, and a lower end of the spring is fixed on an upper surface of the pressing plate. In the process of collecting methane in the gas storage bag, because the spring is always in a compressed state, the pressing plate always presses a topmost gas storage bag. Along with expanding of the gas storage bag, the pressing plate rises. However, the volume of the gas storage bag is always minimum depending on elasticity of the spring. Therefore, space occupied by the gas storage bag is reduced. In another embodiment, the pressing mechanism includes a nut and a ring-shaped pressing plate. The pressing plate is sleeved on the post. The nut is threadedly connected on the upper portion of the post. A lower surface of the pressing plate always presses the topmost gas storage bag under the gravity, and the pressing plate moves upward along with expanding of the gas storage bag and moves downward along with shriveling of the gas storage bag. Therefore, space occupied by the gas storage bag is reduced. The nut can prevent the pressing plate detaching from the post. In the methane production system of the methane tank, the upper portion of the post is further connected to a drive mechanism that can drive the upper portion to rotate. The drive mechanism includes a support, a coupling and a motor. The motor is fixed on the base through the support. The coupling is positioned between an output -5shaft of the motor and the upper portion of the post. Position of the nut on the post can be adjusted properly by the drive mechanism. Therefore, whenever the gas storage bag has gas or no gas, the spring is always in a compressed state to apply pressure to the gas storage bag. Forward and reverse rotation of the post can be achieved by controlling forward and reverse rotation of the motor through the coupling, thereby adjusting the position of the nut on the post. The post rotates forward as the motor rotates forward, and the nut rises. The post rotates reversely as the motor rotates reversely, and the nut falls. In another embodiment, the drive mechanism includes a driving gear, a driven gear, a chain and a motor. The driving gear is fixed on the output shaft of the motor. The driven gear is fixed on the upper portion of the post. The chain is set between the driving gear and the driven gear. The motor is fixed on the base through a fixing plate. Forward and reverse rotation of the post can be achieved by controlling forward and reverse rotation of the motor through the chain, thereby adjusting the position of the nut on the post. The driving gear rotates forward as the motor rotates forward to drive the post to rotate forward through the chain, and the nut rises. The driving gear rotates reversely as the motor rotates reversely to drive the post to rotate reversely through the chain, and the nut falls. In the methane production system of the methane tank, the connecting valves are electromagnetic valves. A first programmable logic controller (PLC) is fixed on the base. The connecting valves and the motor are connected to the first PLC through wires. In the process of collecting methane in the gas storage bag, the first PLC can automatically control one electromagnetic valve to open and import the methane in a gas storage bag that is in communication with the one -6electromagnetic valve. When the gas storage bag is full, the first PLC controls the one electromagnetic valve to close and a next electromagnetic valve to open. The first PLC opens and closes the electromagnetic valves in cycle until the gas storage bags on the post are full of methane. Forward and reverse rotation of the motor can be controlled through the first PLC. In the methane production system of the methane tank, the control mechanism includes a second PLC and a temperature sensor. The temperature sensor is fixed on the bottom of the tank body. The second PLC is fixed on an inner wall of the tank body. The temperature sensor and the electric heating pipe are connected to the second PLC through wires. The temperature sensor can obtain a temperature in the methane tank. A temperature is set in the second PLC. When a temperature obtained by the temperature sensor is smaller than the temperature set in the second PLC, the second PLC can control electricity of the electric heating pipe through the wires to heat the methane tank, thereby increasing the temperature in the methane tank. In the methane production system of the methane tank, a stirring device that can stir in the tank body is provided in the tank body. The stirring device includes a stirring motor, a stirring rod and a fixing rod. An end of the fixing rod is fixed on an inner wall of the methane tank. The stirring motor is fixed on another end of the fixing rod, and an output shaft of the stirring motor faces the bottom of the tank body. The stirring rod is fixed on the output shaft of the stirring motor. The stirring motor is connected to the second PLC through wires. The stirring motor is connected to an external power source and is turned on or off by control of the second PLC. The stirring motor can stir liquid material in the methane tank to make a temperature of the liquid -7material more uniform and make the liquid material better absorb heat in the methane tank, thereby increasing methane yield in the methane tank. In the methane production system of the methane tank, an installation hole is defined in the cover. The connecting pipe extends through and fixed in the installation hole. A seal ring is fixed on the connecting pipe, and the seal ring is in the installation hole. The seal ring is firstly sleeved on the connecting pipe, and then the connecting pipe with the seal ring is installed in the installation hole, and finally clay is applied to the installation hole for sealing. This achieves sealing between the connecting pipe and the cover. In the methane production system of the methane tank, the methane tank further includes a solar panel and a battery on the tank body. The battery is connected to the solar panel. The electric heating pipe, the temperature sensor, the first PLC and the second PLC are connected to the battery. The solar panel absorbs solar energy and transfers the solar energy into electric energy and stores the electric energy in the battery, so that the battery can continuously supply power to electrical equipments of the methane tank. Compared with the prior art, the methane production system of the methane tank has the following advantages: 1. The present invention successively stacks a number of gas storage bags on the post, places the pressing plate on the gas storage bag, and connects the pressing plate with the compressed spring to make the pressing plate always press a topmost gas storage bag. Under the effect of the spring, the pressing plate moves upward along with expanding of the gas storage bag, and move downward along with the shriveling of the gas storage bag. The volume of the gas storage bag is always minimum depending on elasticity of the spring. -8- Therefore, space occupied by the gas storage bag is reduced. 2. The present invention collects methane using gas storage bag. This is convenient for manufacture and cost is low. Further, tightness is good and it is easy to carry. 3. The present invention sets the temperature sensor and the second programmable logic controller in the methane tank to automatically control the temperature in the methane tank. Therefore, the methane yield of the methane tank is assured. 4. The present invention stirs the methane tank using the stirring motor to make temperature of liquid material more uniform, and meanwhile make the liquid material better absorb heat in the methane tank thereby increasing methane yield in the methane tank. 5. The present invention fixes the solar panel on the methane tank. The solar panel absorbs solar energy and transfers the solar energy into electric energy and stores the electric energy in the battery, so that the battery can continuously supply power to electrical equipments of the methane tank. This greatly reduces the power burden for the system. 6. The present invention controls open and close of the connecting valve using the first PLC to achieve continuous transmission of methane to the gas storage bag. BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic view of a methane produce system according to the present invention. FIG. 2 is a schematic view of a methane tank according to the present invention. FIG. 3 is a schematic view of a storage device according to the present invention. _9_ FIG. 4 is a schematic view of a gas storage bag according to the present invention. FIG. 5 is a schematic view of a drive mechanism according to a third embodiment. Drawings show a methane tank 1, a tank body la, a cover 1b, a connecting pipe 2, an electric heating pipe 3, a base 4, a gas storage bag 5, a first bag body 5a, a second bag body 5b, a gas storage chamber 5c, a post 6, an air pipe 7, a connecting valve 8, a main pipe 9, a bearing housing 10, a pressing plate 11, a spring 12, a nut 13, a motor 14, a coupling 15, a support 16, a first PLC 17, a second PLC 18, a temperature sensor 19, a stirring motor 20, a stirring rod 21, a fixing rod 22, a solar panel 23, a battery 24, a driving gear 25, a driven gear 26 and a chain 27. DETAILED DESCRIPTION Example embodiments will now be described more fully with reference to the accompanying figures. However, the present invention will not be limited to these embodiments. Embodiment 1 As shown in FIGs. 1 and 2, a methane production system of a methane tank includes a methane tank 1. The methane tank 1 include a tank body la that can produce methane, a cover lb and a connecting pipe 2 pivotedly connected to an upper part of the tank body la. An electric heating pipe 3 is set in the tank body la. An end of the connecting pipe 2 extends through the cover lb and is in communication with the tank body la, and the connecting pipe 2 and the cover lb keep airtight. Other end of the connecting pipe 2 is connected to a storage device. A control mechanism that can -10automatically turn on the electric heating pipe 3 when a temperature in the tank body la is smaller than a set value is set in the tank body 1 a. Specifically, an installation hole is defined in the cover lb. The connecting pipe 2 extends through and fixed in the installation hole. A seal ring is fixed on the connecting pipe 2. The seal ring is in the installation hole. The seal ring is firstly sleeved on the connecting pipe 2, and then the connecting pipe 2 with the seal ring is installed in the installation hole, and finally clay is applied to the installation hole for sealing. This achieves sealing between the connecting pipe 2 and the cover lb. As shown in FIG. 3, the storage device includes a base 4, and a number of gas storage bags 5 having gas storage chambers 5c. A post 6 extends from the base 4 along a vertical axis. A mid portion of the gas storage bag 5 defines an installation hole. The gas storage chamber 5c and the installation hole keep airtight. An air pipe 7 is hermetically fixed on the gas storage bag 5 and is in communication with the gas storage chamber 5c. The gas storage bags 5 are successively sleeved on the post 6. The air pipes 7 connect the gas storage chambers 5c of the gas storage bags 5 in series using a pipe structure. A pressing mechanism is sleeved on the post 6 and is used for pressing the gas storage bags 5 on the base 4. The gas storage bags 5 sleeved on the post 6 of the base 4 forms a stacked status. The methane produced by the methane production system can be collected and stored by flowing into the gas storage bag 5 through the pipe structure and the air pipe 7, which is convenient. The gas storage bag 5 on the post 6 can be pressed towards the base 4 by the pressing mechanism, which minimizes volume of the gas storage bags 5, thereby reducing space occupied by the gas storage bags 5. -11- As shown in FIG. 4, the gas storage bag 5 includes a ring-shaped first bag body 5a and a ring-shaped second bag body 5b. The first bag body 5a and the second bag body 5b are attached firmly to each other, and inner and outer edges of the first bag body 5a are hermetically connected to inner and outer edges of the second bag body 5b. A mid ring-shaped hole of the first bag body 5a and a mid ring shaped hole of the second bag body 5b cooperatively form the installation hole. A chamber between the first bag body 5a and the second bag body 5b forms the gas storage chamber 5c. The installation hole is good for the gas storage bag 5 sleeved on the post 6 to position the gas storage bag 5. The methane can be stored in the gas storage chamber 5c. An end of the air pipe 7 is hermetically fixed to the first bag body 5a or the second bag body 5b and the air pipe 7 is in communication with the gas storage chamber 5c. Another end of the air pipe 7 is a closed end. A pipe body of the air pipe 7 defines a connecting hole in communication with an internal channel of the air pipe 7. The end of the air pipe 7 in communication with the gas storage bag 5, the another end being a closed end, and the connecting hole defined at a side of the pipe body of the air pipe 7 are good for better stacking the gas storage bags 5. The connecting hole is good for importing external methane in the gas storage chamber 5c. Specifically, the gas storage bag 5 is made of plastic material. This is convenient for manufacture and cost is low. Further, tightness is good and it is easy to carry. Specifically, the pipe structure includes a number of connecting valves 8 and a main pipe 9. An end of the main pipe 9 has a communicating port. The communicating port is in communication with the connecting pipe 2 and is provided for importing the methane. Another end of the main pipe 9 is a closed end. An end of the -12connecting valve 8 is hermetically connected to the connecting hole of the air pipe 7. Another end of the connecting valve 8 is hermetically fixed to a pipe body of the main pipe 9 and is in communication with an internal channel of the main pipe 9. After entering the communicating port of the main pipe 9, the methane enters the gas storage bag 5 through the connecting valve 8 and the air pipe 7. The connecting valve 8 is used for connecting and disconnecting the main pipe 9 to the gas storage bag 5, which achieves an automatic control of transporting and collecting methane. Specifically, the connecting valves 8 are electromagnetic valves. A first programmable logic controller (PLC) 17 is fixed on the base 4. The connecting valves 8 are connected to the first PLC 17 through wires. In the process of collecting methane in the gas storage bag 5, the first PLC 17 can be set to automatically control one electromagnetic valve to open and import the methane in a gas storage bag 5 that is in communication with the one electromagnetic valve. When the gas storage bag 5 is full, the first PLC 17 controls the one electromagnetic valve to close and a next electromagnetic valve to open. The first PLC 17 opens and closes the electromagnetic valves in cycle until the gas storage bags 5 on the post 6 are full of methane. More specifically, a bearing housing 10 is fixed on the base 4. The bearing housing 10 has a bearing inside. The post 6 includes a lower portion and an upper portion fixed on the lower portion. The lower portion extends through and fixed on an inner ring of the bearing. The upper portion has external threads. This design makes the post 6 rotate with the inner ring of the bearing on the base 4. The pressing mechanism includes a pressing plate 11, a spring 12 and a nut 13. A mid portion of the pressing plate 11 defines a lifting hole. The post 6 extends through the lifting hole of the pressing plate 11. The nut -13- 13 is threadedly connected on the upper portion of the post 6 and the spring 12 is in a compressed state. An upper end of the spring 12 is fixed on the nut 13, and a lower end of the spring 12 is fixed on an upper surface of the pressing plate 11. In the process of collecting methane in the gas storage bag 5, because the spring 12 is always in a compressed state, the pressing plate 11 always presses a topmost gas storage bag 5. Along with expanding of the gas storage bag 5, the pressing plate 11 rises. However, the volume of the gas storage bag 5 is always minimum depending on elasticity of the spring 12. Therefore, space occupied by the gas storage bag 5 is reduced. The upper portion of the post 6 is further connected to a drive mechanism that can drive the upper portion to rotate. Position of the nut 13 on the post 6 can be adjusted properly by the drive mechanism. Therefore, whenever the gas storage bag 5 has gas or no gas, the spring 12 is always in a compressed state to apply pressure to the gas storage bag 5. Specifically, the drive mechanism includes a support 16, a coupling 15 and a motor 14. The motor 14 is fixed on the base 4 through the support 16. The coupling 15 is positioned between an output shaft of the motor 14 and the upper portion of the post 6 and the motor 14 is connected to the first PLC 17 on the base 6 through wires. Therefore, the first PLC 17 can control forward and reverse rotation of the motor 14. Forward and reverse rotation of the post 6 can be achieved by controlling the forward and reverse rotation of the motor 14 through the coupling 15, thereby adjusting the position of the nut 13 on the post 6. The post 6 rotates forward as the motor 14 rotates forward, and the nut 13 rises. The post 6 rotates reversely as the motor 14 rotates reversely, and the nut 13 falls. The control mechanism includes a second PLC 18 and a temperature sensor 19. The temperature sensor 19 is fixed on the -14bottom of the tank body la. The second PLC 18 is fixed on an inner wall of the tank body la. The temperature sensor 19 and the electric heating pipe 3 are connected to the second PLC 18 through wires. The temperature sensor 19 can obtain a temperature in the methane tank 1. A temperature is set in the second PLC 18. When a temperature obtained by the temperature sensor 19 is smaller than the temperature set in the second PLC 18, the second PLC 18 can control electricity of the electric heating pipe 3 through the wires to heat the methane tank 1, thereby increasing the temperature in the methane tank 1. A stirring device that can stir in the tank body la is provided in the tank body la. The stirring device includes a stirring motor 20, a stirring rod 21 and a fixing rod 22. An end of the fixing rod 22 is fixed on an inner wall of the methane tank 1. The stirring motor 20 is fixed on another end of the fixing rod 22 and an output shaft of the stirring motor 20 faces the bottom of the tank body la. The stirring rod 21 is fixed on the output shaft of the stirring motor 20. The stirring motor 20 is connected to the second PLC 18 through wires. The stirring motor 20 is connected to an external power source and is turned on or off by control of the second PLC 18. The stirring motor 20 can stir liquid material in the methane tank 1 to make a temperature of the liquid material more uniform and make the liquid material better absorb heat in the methane tank 1, thereby increasing methane yield in the methane tank 1. In this embodiment, the methane tank 1 further includes a solar panel 23 and a battery 24 on the tank body la. The battery 24 is connected to the solar panel 23. The electric heating pipe 3, the temperature sensor 19, the first PLC 17 and the second PLC 18 are connected to the battery 24. The solar panel 23 absorbs solar energy -15and transfers the solar energy into electric energy and stores the electric energy in the battery 24, so that the battery 24 can continuously supply power to electrical equipments of the methane tank 1. Embodiment 2 Structures and principles of this embodiment are substantially the same as those of the embodiment 1, and differences are that: in the embodiment 1, the pressing mechanism includes the pressing plate 11, the spring 12 and the nut 13; in the embodiment 2, a pressing mechanism includes the nut 13 and a ring-shaped pressing plate 11. The pressing plate 11 is sleeved on the post 6. The nut 13 is threadedly connected on the upper portion of the post 6. A lower surface of the pressing plate 11 always presses the topmost gas storage bag 5 under the gravity, and the pressing plate 11 moves upward along with expanding of the gas storage bag 5 and moves downward along with shriveling of the gas storage bag 5. Therefore, the volume of the gas storage bag 5 is always minimum and space occupied by the gas storage bag 5 is reduced. The nut 13 can be turned to an upper end of the post 6 by hand, thereby preventing the pressing plate 11 detaching from the post 6. Embodiment 3 Structures and principles of this embodiment are substantially the same as those of the embodiment 1, and differences are that: in the embodiment 1, the drive mechanism includes the support 16, the coupling 15 and the motor 14; in the embodiment 3, as shown in FIG. 5, a drive mechanism includes a driving gear 25, a driven gear 26, a chain 27 and the motor 14. The driving gear 25 is fixed on the output shaft of the motor 14 using flat key. The driven gear 26 is fixed on the upper portion of the post 6 using flat key. The chain 27 is set -16between the driving gear 25 and the driven gear 26. The motor 14 is fixed on the base 4 through a fixing plate. The fixing plate is fixed on the base 4 by screws. Forward and reverse rotation of the driving gear 25 can be achieved by controlling forward and reverse rotation of the motor 14, thereby achieving forward and reverse rotation of the post 6 through the chain 27 and finally adjusting the position of the nut 13 on the post 6. The driving gear 25 rotates forward as the motor 14 rotates forward to drive the post 6 to rotate forward through the chain 27, and the nut 13 rises. The driving gear 25 rotates reversely as the motor 14 rotates reversely to drive the post 6 to rotate reversely through the chain 27, and the nut 13 falls. The foregoing description of the embodiments has been provided for purposes of illustration and description of spirit of the present invention. Various alternations, modifications, or equivalents to the technical solutions of the disclosed embodiments can be obvious to those skilled in the art and can be included within the scope of the disclosure. Although the disclosure mostly uses terms such as methane tank 1, tank body la, cover 1b, connecting pipe 2, electric heating pipe 3, base 4, gas storage bag 5, first bag body 5a, second bag body 5b, gas storage chamber 5c, post 6, air pipe 7, connecting valve 8, main pipe 9, bearing housing 10, pressing plate 11, spring 12, nut 13, motor 14, coupling 15, support 16, first PLC 17, second PLC 18, temperature sensor 19, stirring motor 20, stirring rod 21, fixing rod 22, solar panel 23, battery 24, driving gear 25, driven gear 26, chain 27, etc. However, possibility of use of other terms is not excluded. Using these terms are more convenient to describe and explain the nature of the present invention. Additional limitations to explanation of these terms disobey the spirit of the present invention. -17-