1CONNECT AND DISCONNECT DEVICE FOR ELECTRIC POWER CELL FIELD OF THE INVENTION The present invention relates generally to electric power cells (batteries), and particularly to a device that can selectively connect and disconnect a terminal of the cell from a terminal of the device, which may be useful for a Sabbath mode of operation. BACKGROUND OF THE INVENTION Electric power cells (or batteries or cells, the terms being used interchangeably throughout) can be used to power many kinds of devices. For example, smart door locks are often powered by AA batteries. The batteries power the printed circuit board or boards, actuators, and sensors on the lock. Jewish people who observe Sabbath laws require a mode in which the lock is powered off in order to comply with Sabbath laws. Simply removing the batteries is not a satisfactory solution to create a certified Sabbath compliant device. SUMMARY OF THE INVENTION The present invention seeks to provide a device that can selectively connect and disconnect a terminal of a battery from a terminal of the device, which may be useful for a Sabbath mode of operation, as described more in detail hereinbelow. The invention is described for use with a lock device, but can be used for other battery-powered devices. There is thus provided in accordance with a non-limiting embodiment of the present invention a device for use with a battery including a connect-disconnect device including a body with open ends, placeable over an end of a battery so that a terminal of the battery exposed at one of the open ends, the connect-disconnect device including a grasping member and a cam protruding from the body, wherein moving of the connect-disconnect device causes the cam to move the battery into and out of electrical connection with a power source contact. The connect-disconnect device may be moved rotationally or linearly, for example. The body may include a Sabbath mode indicator and a weekday mode indicator. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: Fig. 1 is a simplified perspective illustration of the exterior of a lock device which is powered by at least one battery, in accordance with a non-limiting embodiment of the present invention; 2Fig. 2 is a simplified front-view illustration of the lock device of Fig. 1, with the battery access cover removed and showing the battery in a Sabbath mode; Fig. 3 is an enlarged front-view illustration of the lock device of Fig. 1 with the battery in the Sabbath mode and clearly showing that the battery terminal has been moved out of electrical contact with a power source contact; Fig. 4 is a simplified front-view illustration of the lock device of Fig. 1, with the battery access cover removed and showing the battery in a weekday mode, in which the battery terminal is in electrical contact with the power source contact; Figs. 5 and 6 are simplified perspective illustrations of the connect-disconnect device assembled on the battery, in respective Sabbath and weekday positions; and Fig. 7 is a simplified perspective illustration of the connect-disconnect device, in accordance with a non-limiting embodiment of the present invention. DETAILED DESCRIPTION Reference is now made to Fig. 1, which illustrates a lock device 10 which is powered by at least one battery, in accordance with a non-limiting embodiment of the present invention. Without limitation, the lock device 10 may include a housing 12 with a removable battery access cover 14 and possibly electrical switch or controls or user interface (all not shown). For a lock device, the device may be provided with a thumb turn 16 for moving a lock bolt associated with a door and the like. Reference is now made to Figs. 2 and 3, which illustrate the lock device 10 of Fig. 1, with the battery access cover removed and showing a battery 18 in a Sabbath mode. Battery 18 may be any kind of electrical power cell, such as but not limited to, AA or AAA cells. In one embodiment of the invention, a connect-disconnect device 20 is assembled on one end of the battery, such as at the end which has a positive terminal 22. As seen in Figs. 5-7, connect-disconnect device 20 may include a body 24 (in this case a cylindrical body) with open ends, which can fit over the end of the battery and leave the positive terminal 22 exposed. The connect-disconnect device 20 may include a grasping member 26, such as a longitudinal spine that radially protrudes out of body 24. Other grasping members may be used, such as a knurled portion, dimples, knobs and many more. The connect-disconnect device 20 may include a cam 28, such as but not limited to, a ramped cam that protrudes from an upper surface 29 of body 24. In use, the connect-disconnect device 20 may be assembled over the end of the battery and leave the positive terminal 22 exposed, as seen in Figs. 5 and 6. In the Sabbath mode, as seen in Figs. 2 and 3, the user has removed the battery access cover to 3gain access to the connect-disconnect device 20. The use can grasp the grasping member and turn connect-disconnect device 20. During turning of connect-disconnect device 20, as seen best in Fig. 3, the cam 28 pushes against a surface 30 of housing 12 of device 10, thereby moving and distancing the positive terminal 22 of battery 18 away from a power source contact 32. (Movement of the battery 18 away from power source contact may compress a spring of the device located at the negative terminal of the battery, for example.) This position may be indicated by a Sabbath mode indicator 34, such as the letter S. Thus, by turning connect-disconnect device 20 to the Sabbath mode, the battery terminal 22 has been moved out of electrical contact with power source contact 32 and the device 10 is now in compliance with the Sabbath laws and can only be used in a mechanical mode and not an electrical mode of operation. The surface 30 against which the cam 28 acts may be any existing surface in or on the housing 12, or may be purposely added or machined and the like to provide the acting surface for cam 28. It is noted the body 24 of connect-disconnect device 20 may be tapered to snugly fit over battery 18 so that connect-disconnect device 20 does not slip around battery when turning connect-disconnect device 20. Alternatively, the body 24 of connect-disconnect device 20 may have a roughened or adhesive inner surface so that connect-disconnect device 20 does not slip around battery 18 when turning connect-disconnect device Reference is now made to Fig. 4, which shows the battery 18 in a weekday mode, in which the battery terminal 22 is in electrical contact with the power source contact 32. In the weekday mode, the user has removed the battery access cover to gain access to the connect-disconnect device 20. The use can grasp the grasping member 26 and turn connect-disconnect device 20. During turning of connect-disconnect device 20, the cam has been moved away from surface 30, so that the positive terminal 22 of battery 18 is in contact with power source contact 32. This position may be indicated by a weekday mode indicator 36, such as the letter W. Thus, by turning connect-disconnect device 20 to the weekday mode, the battery terminal 22 has been moved into electrical contact with power source contact 32. Other connect-disconnect devices are contemplated to carry out the invention. For example, the connect-disconnect device may move the battery linearly and have a tongue, latch or ratchet and the like (all referred to as a cam) that can catch onto an inner surface of the housing and lock the battery in a position in which the battery terminal 22 has been 4moved out of electrical contact with power source contact 32, and which can be released to bring the battery terminal 22 into electrical contact with power source contact 32.