A drive chain, especially for use in mining machines. Technical Field The invention relates to a drive chain, for use in mining machines such as chain conveyors or chain-driven extraction machines in particular, of the type described in the classifying part of Claim 1. Prior Art In underground mining, chain drives are conventionally used for high performance machinery, in particular scraper chain conveyors and chain-driven extraction machines, especially coal planes but also coal cutters, which are used in high-yield extraction operations. Whereas in the past normal size round link chains made of round wire were used for the coal planes and drag chains, the industry has long since progressed to the use of round link chains with links specially designed to meet the constantly increasing demands in terms of power and operational performance placed upon the drive chains used in these mining machines. To make it possible to use stronger chains in existing scraper chain conveyors and thus meet the requirement for higher performance, a prior art suggestion is to flatten off the vertical links of the chains on their parallel longitudinal shanks and thus, without reducing the cross-section of their structural height, to reduce them to a size similar to that of a standard round-link chain of lesser strength (DE 32 34 137 C3, DE 36 15 734 C2). Furthermore, much effort has been expended t6 improve the running and load-bearing quality of drive chains on which heavy operational demands are made, especially in relation to their extension properties and their co-operation with the drive chain wheels. Thus for example it is a known practice to use special links, broadened at their chain-link nose-sections, for the horizontal links of chains on which heavy demands are placed, thereby reducing the specific surface pressure as the chain runs round the chain wheels and hence also the wear on the co operating surfaces of the horizontal chain links and the chain wheel teeth (DE 32 35 474 C2). In these drive chains, conventional round wire chain links of welded design are used for the vertical chain links. 1 The invention The invention is based upon a drive chain of round link design, as disclosed in the above mentioned DE 32 34 474 C2, in which the links which co-operate with the chain drive - meaning in particular with the drive chain wheel - and which receive the power of the chain drive, consist of power transfer chain links made from flat links specially designed for these purposes, which are inextensibly connected via connecting elements linked within them. It is a primary object of the invention to design this drive chain in such a way that without excessively increasing the dimensions of its chain links it is especially suited to accommodate increased drive power and corresponding transmission of high tension forces. This object is solved in the invention by the characteristic features of Claim 1. For those chain links which co-operate with the chain drive - meaning in particular with the drive chain wheel - and via which the drive force is thus supplied to the chain, and which in general form the horizontal links of the drive chain, the drive chain according to the invention uses chain links whose rigidity, dimensions and co-operation with the drive chain wheel or the like are optimised, and which for the purpose of rigidity do not need to have greater length and width dimensions than the round chain links of a standard chain of lower rigidity. Here, the drive chain according to the invention combines the advantages of the previously known drive chain as per DE 32 34 474 C2, in respect of load-bearing performance and co-operation with the chain drive (drive chain wheel), with extra advantages which arise on the one hand from the design of the nose sections of the power transfer (horizontal) chain links and on the other from the fact that the chain noses of the loop-type connecting elements linked in the interior openings of the power transfer chain links are supported over a comparatively large area on the inner surfaces of the thickened noses of the power transfer chain links, and indeed on both sides of the swivel sectors formed by cross-section recesses in the noses of the power transfer chain links, thus achieving transfer of the drive and chain-tension forces over a comparatively large surface, while guaranteeing sufficient angular freedom of movement for the chain links. Accordingly, on the power transfer chain links the build-ups of material produced by their nose 2 thickenings serve on the one hand to improve the transmission of the drive power of the chain wheel or the like to the chain, and on the other hand to improve the transfer of tension forces between the chain links flexibly linked in one another, with the result that the drive chain offers especially good load-bearing and operational performance when used with high performance drives. In order to guarantee sufficient lateral swivel movement for the power transfer chain links flexibly linked in one another, it is advisable for their nose or swivel sectors to be designed to cover an arc angle of approximately 400 to 50*, for example 45*. In this design the central point of the swivel sectors lies within the interior opening of the power transfer chain links on their central longitudinal axis. The power transfer chain links should preferably have an approximately rectangular shape. It is advisable for them to be designed symmetrically to all three chain link central axes or rather to the corresponding central planes. It is advisable to design the thickness of the shanks of the power transfer chain links in such a way that it increases from the central area of the shank towards the thickened portion of the chain noses. In addition it is recommended that said nose thickenings are designed in such a way on both sides of the swivel sectors that they increase towards the outer nose ends and there run into the shanks of the power transfer chain links. All these design measures contribute to the increase in rigidity of the power transfer chain links and to the improvement of their characteristics for accommodating the drive power. At the same time a favourable weight/dimension ratio for these chain links is achieved, with a distribution of material which facilitates accommodation of the drive power. The rigidity of the loop-type connecting elements linking the power transfer chain links and linked flexibly in these is of course adjusted to the required chain tensile strength and accordingly to the rigidity of the power transfer chain links. At the same time the connecting elements may have various different cross-sections, for example an oval cross-section, whose longer cross-section axis runs perpendicular to the central longitudinal axis of the power transfer chain links. At the same time the loop-type connecting elements may be given a constant cross-section throughout but may also be thickened in cross-section in their nose area. The power transfer chain links which during operation of the chain principally form the horizontal links of the chain may have various different cross-sections in the area of their swivel sectors situated in the middle of the chain nose, but preferably have a rounded, oval or elliptical cross 3 section whose larger cross-section axis lies in the central longitudinal axis of the chain link. In a further advantageous design the power transfer chain links are designed so that their noses are equipped with concavities or hollows on the outsides of their nose thickenings, on both sides of the profile axis running through the centre of the chain noses and the shanks. At the same time it is advisable for the nose thickenings of the power transfer chain links to have a protrusion, roughly hammer-headed in shape when seen from the front, added onto their shanks in the outer area. With the design of the power transfer chain links according to the invention, damaging elongations of these chain links due to plastic deformation during operation of the drive chain are also considerably reduced, thus enabling a larg-ly stretch-free drive chain to be achieved. The matching of the mutually supporting surfaces of the power transfer chain links and the connecting elements linked in them leads to a large-surface transfer of the chain power to these links and thus reduces the friction and deformation forces placed upon these during use of the drive chain. The result is that the invention creates a drive chain which, using chain links whose dimensions or weight are not excessive, combines suitability for the transfer of high performance drive with favourable operational characteristics and which is further distinguished by high durability in use. Further advantageous features of the drive chain according to the invention are indicated in the individual claims and furthermore are seen in the following description of the embodiments shown in the drawing: Brief description of the drawings: Figure 1 shows a plan of a partial longitudinal cross-section of a drive chain according to the invention; Figure 2 shows the drive chain as per Figure 1 in a side view; Figure 3 shows a plan of an individual power transfer chain link from the drive chain shown in Figure 1 and 2; Figure 4 shows the chain link shown in Figure 3 in a side view in the direction of the arrow 4 IV in Figure 3, one of the two chain noses being shown in a vertical section along the chain link central axis; Figure 4a shows the same as Figure 4 but with a slightly altered version of the power transfer chain link; Figure 5 shows a view of the nose of the power transfer chain link in the direction of the arrow V in Figure 3; Figure 6 shows in the cross-section view VI-VI in Figure 3 a vertical cross-section through one of the two shanks of this chain link, in the actual centre of the shank; Figure 7 shows in larger scale a partial view from above corresponding to Figure 3 of the power transfer chain link with connecting element linked in it; Figure 8 shows a side view of a loop-type connecting element which is usable in the drive chain according to the invention; Figure 9 shows, in the representation according to Figure 7, the transfer of power from the power transfer chain link to the connecting element linked in it. Preferred embodiments of the invention For the understanding of the invention reference is made to DE 32 35 474 C2 mentioned above, whose content is incorporated by reference. As mentioned, the drive chain according to the invention is primarily designed for use in chain conveyors, particularly scraper chain conveyors and chain-driven mining extraction machines such as coal planes in particular, in which the drive of the continuous chain, as is well-known, is effected via drive chain wheels or the like. In this case those links of the drive chain via which tle drive force or drive torque from the chain drive - or its chain wheel - are supplied to the chain are called power transfer chain links, whilst the loop-type links flexibly connecting these chain links are referred to below as connecting elements. In general the power transfer chain links are the horizontal chain links of the drive chain and consequently the connecting elements are its vertical links. The drive chain, of which only a short partial area of its length is shown in Figures 1 and 2 consists alternately of the power transfer chain links 1 and the connecting elements 2 linked flexibly within them. The chain links 1 co-operating with the drive chain wheels and transferring 5 the drive force or drive torque as tensile force to the drive chain are designed as flat links and have a roughly rectangular shape with rounded corner areas 3, as can be seen from Figure 3. The chain links 1 co-operating with the drive chain wheel or its toothed starwheel to transfer the power are the horizontal links, and their connecting elements 2 the vertical links of the drive chain, whose chain axis is indicated in Figures 1 and 2 by A. The chain links 1, with their interior openings 4, have a chain link pitch tI and the connecting elements 2 with their interior openings 5 have a chain link pitch t2. The chain link pitches t 1 and t2 may be of the same size or may also differ from one another, the chain link pitch tl being larger than the chain link pitch t2. Details of the design of the power transfer chain links 1 may be seen in Figures 3 to 7 in particular. The parallel shanks 6 of the chain links 1 have chain noses 7 connecting their ends, which in the conventional way extend over the whole width of these chain links. The central longitudinal axis is defined by L, the transverse middle axis running at right angles to this across the width of the chain link by B and the central axis running perpendicular to these axes L and B and defining the central flat plane of the chain link 1, by C. The chain links 1 are, as can be seen, designed symmetrically to all three central axes L, B and C, i.e. to all three chain link planes standing vertically upon one another. On each chain link 1 the two end noses 7 are identical and have thickened noses. The measure of altitude of the chain noses 7 in the area of their outer ends to which the shanks 6 are connected, is indicated in Figure 5 by H. A swivel sector 8 formed by a cross-section concavity in the nose is moulded onto each chain link 7, centrally and symmetrically to the chain link central axis L, extending from the interior opening 4 of the chain link 1 to the outer face of the relevant chain nose 7 and running conically from the interior opening to the outer face of the chain nose 7, while the swivel sectors 8, which constitute the linking points for the connecting elements 2, are bordered by the border lines 9 diverging outwards from one another, which meet within the interior opening 4 at an imaginary central point M. Accordingly, the swivel sectors 8 on each chain nose 7 have an angle of opening between the two border lines 9 which determines the extent of lateral swivelability of the chain links and connecting elements 1 and 2 linked flexibl> in one another. It is advisable for the angle to be around 40-50', preferably around 450, as shown in the drawing. The imaginary central points M of the swivel sectors 8 lie on the central longitudinal axis L of the chain link 1. The swivel 6 sectors 8 form the mating points on the chain links 1 for the linked connecting elements 2. The chain noses 7 have a rounded cross-section or an oval cross-section, or, as shown in Figure 4 by hatching, a somewhat elliptical cross-section 10 in the area of the swivel sectors 8, whose arc contour 10' lying on the side of the interior opening 4 has a larger radius of curvature than its arc contour 10" located on the outer face of the chain nose 7. The larger cross-section axis of the elliptical cross-section 10 of the chain nose 7 on the arc area of the swivel sectors 8 lies in the central axis L of the chain links. On both sides of the arc-shaped swivel sectors 8 each chain nose 7 has thickenings 11, which increase towards the outside of the nose ends and at the outside of the nose ends terminate in the shanks 6 of the chain links 1. As Figures 3, 5 and 7 in particular show, the noses 7 of the chain links 1 have exterior concavities on their thickenings 11, which form hollows 12 on the outer faces of the chain noses 7 which are positioned symmetrically to one another on both sides of the central axis C (Figure 5) and which at the rear terminate in the shanks 6 (Figures 3 and 7). The moulding of these hollows 12 gives the nose thickenings 11 a protrusion 13, of hammer-head shape when seen from the front, in the exterior area of the shanks 6, as can be seen from Figure 5, its width measurement defining the height measurement H of the chain noses 7, or of their thickenings 11. The exterior surfaces of these hammerhead-like protrusions 13 together with the exterior surface of the chain noses on the central area of their swivel sectors 8 form the outer faces of the chain links 1, which constitute bearing surfaces for the drive chain wheel. As can especially be seen in Figure 4, these outer faces or bearing surfaces 14 are convexly rounded outwards forming an arc over the whole height measurement H. In the central area of the swivel sectors 8 their outer arc-shaped borderlines follow the arc contour 10" in the plane of the outer faces or support surfaces 14 located either side of the swivel sector. On each power transfer chain link 1 the two parallel chain link longitudinal shanks 6 are smallest in cross-section centrally, i.e. in the area of the transverse central axis B, and as shown in Figure 6 this may have a roughly rectangular cross-section 6' with rounded-off corners 6", in which case it is advisable for the areas 6"' lying on the outer and inner sides of the shank to be designed slightly convexly arched. Starting from this central cross-section 6' the thickness of the shanks 7 6 of the chain links 1 increases towards the nose thickenings 11, producing the arc 15 shown in Figure 4 on the upper and lower side of the shanks 6. On each chain link 1 the two shanks 6 thus run into the nose thickenings 11 and the hammerhead-shaped protrusions 13, their cross-section constantly increasing. Figure 4a shows a slightly altered design in which the arc path 15 of the shanks 6 runs into a straight path 15', laterally removed from the central shank cross-section 6', and itself running into the ends of the shanks at the nose thickenings 11. The above-described design of the power transfer chain links 1 produces a build-up of material on these in the area of the chain noses 7 on either side of the swivel sectors 8, while the width or height of the nose thickenings 11 or the hammerhead-shaped protrusions 13 forming them may correspond, at least approximately, to half the length of a nose 7. This results in a high degree of rigidity for the chain links, without requiring that the exterior dimensions of the chain links 1, i.e. their length and width vis-a-vis the standard round chain links, be increased. At the same time this design of the power transfer chain links 1 produces large bearing surfaces between their noses and the co-operating tooth-face surfaces of the toothed drive chain wheels, so that the high performance drive of the chain drive can be transmitted without excess surface pressure on the chain links 1. Damaging deformations of the power transfer chain links in operation are largely avoided, also excessive wear of same in co-operation with the drive chain wheel 1. The connecting elements 2 linked, horizontally, vertically and flexibly movable, in the chain links 1 on their swivel sectors 8, and which can be designed as welded chain links in the conventional way, may have differing cross-sections. Thus they can be provided with a constant cross-section throughout, or, may however as shown in Figure 8 be thickened in cross-section in the area of their two arc-shaped chain noses 2'. In the design according to Figures 1 to 7 the connecting elements 2 have an almost oval flat cross-section 2' with a cross-sectional width b formed by the longer cross-section axis, which is only smaller than the interior width b' of the interior opening 4 of the chain links 1 by the required amount of free play, as can be seen from Figure 7. Accordingly, the connecting elements 2 linked flexibly in the interior openings 4 of the chain links 1 are supported, on either side of the swivel sectors 8 running out into the interior opening, on 8 support and bearing surfaces 16 running across the central longitudinal axis L, which in the example shown are formed by the rounded surfaces linking them to the inner sides of the shanks 6. Obviously, the support surfaces 16 and the surfaces of the arc-shaped nose sections of the connecting elements supported upon them are matched to one another in design, in order to achieve favourable bearing ratios here during tension force transfer between the chain links. Independently of the cross-section shape of the connecting elements 2 their nose width b is thus larger in dimension than the width of the opening of the swivel sectors 8 on the interior opening 4 of the power transfer links of the chain links 1. Figure 9 schematically portrays the transfer of power between the chain links 1 and 2 when the drive chain according to the invention is used. The drive force FK supplied during chain drive by the chain wheel to the outer face of the chain link 1 is introduced via the nose thickenings of the chain link directly into its shank 6. During this the tension forces of the drive chain are transferred to the power transfer chain links 1 by the fact that the connecting elements 2 are supported on the inner bearing surfaces of the power transfer chain links 1, and indeed on both sides of the swivel sectors 8 running into the interior opening 4, thus also in the area of the cross section reinforcements effected by the nose thickening of the chain links 1. This produces high tension force components Fy for the tension force operating in the direction of the chain axis, and only relatively small transverse power components Fx across the chain axis, so that here there are favourable power transfer ratios. Damaging torsion loads upon the power transfer chain links 1 via the transverse forces Fx are avoided. It is advisable, especially when the connecting elements 2 as shown for example in Figure 8 are equipped with nose reinforcements, to dimension the chain link pitch tl of the power transfer chain links 1 larger than the chain link pitch t2 of the connecting elements 2. It can be seen from Figure 7 that the connecting elements 2 have a cross-section width b corresponding approximately to the exterior width of the swivel sectors 8 or the interior width b' of the interior opening of the chain links 1. Starting from the radial borderlines 9 delimiting the swivel sector 8 on either side of the central longitudinal axis L each chain nose 7 increases in the direction of the arrow S in 9 Figure 7 in the area of the hollows 12 towards the nose thickenings 11 on the two nose ends, the ratios being selected in such a way that the connecting elements too possess sufficient freedom of movement across the chain axis. Through the arc 15 of the shanks 6 of the chain links 1 the favourable build-up of material is produced in the exterior areas of the chain noses 7, which can thus in these areas contain the nose thickenings with the hammer-head shaped protrusions 13 bordered, towards the swivel sectors 8, by the hollows. It is advisable for the shanks 6 of the power transfer chain links 1 to be given rounded outer surfaces. Said hollows 12 run into the swivel sectors 8 and on the inside into the supporting surfaces 16 for the linked connecting elements 2. In general the power transfer chain links 1 are designed as single-part chain links, for example as forged chain links. The power transfer chain links preferably consist of a hard-wearing, non weldable steel, for example a high-tensile heat-treatable steel, a two-phase steel or a stainless steel. On the other hand there is also the possibility of designing the chain links 1 in multiple parts, i.e. from several parts, connected by welding for example. Where the drive chain according to the invention is used in a scraper chain conveyor the power transfer chain links 1 may be the scraper carriers. The power transfer chain links 1 and/or the connecting elements 2 may also be designed in the manner of chain joints which may be opened for linking into the chain assembly. The chain links 1 and 2 can be inductively coated. There is also the possibility of manufacturing the co-operating surfaces of the chain links 1 and 2 and/or of the chain links 1 and of the chain drive or chain wheel partially induction- or flame-hardened. The mating materials of the chain links 1 and 2 flexibly linked in one another, on the tooth faces of the chain wheel and/or on the faces of the power transfer chain links may also feature a hardness difference which avoids a cycle of cold welding and subsequent tearing apart of the co-operating surfaces. There is also the possibility of hardening - among the surfaces co-operating in the chain link points - at least the surfaces of the power chain links 1 and /or the power transfer surfaces of the chain wheel by the application of layers of hard material, preferably in basically conventional ways using either plasma nitride or plasma boron. Clearly, the invention is not limited to the design example described above, but may undergo many alterations without departing from the framework of the invention. 10