Device for document alignment and feed * * * * * DESCRIPTION Technical field of the invention The present invention refers to a device for document alignment and feed. More specifically, the present invention refers to a device for the alignment and feed of documents for printers, such as for example, sheets of paper or similar. Brief description of the state of the art A device for the alignment and feed of documents is mounted on a printer mainly destined for banking use, by Olivetti-Tecnost S.p.A., and has been described in certain aspects thereof in US patent 5,947,471 issued to Ing. Olivetti & C. S.p.A. The detection of the presence of a document is performed by light sources (leds) and photodetectors arranged on a same part of the support surface, and by optical fibres which, if the document is absent, re-transmit the light beams from the light sources to the photodetectors. The optical fibres are of the type commonly found on the market and are inserted in seats set on the opposite side of the support surface. The device is very efficient. However it presents the risk that the fibres can be pulled out from their seats. For this reason device testing must be performed at the same time as printer testing, and it is not possible to test the device in advance, thereby avoiding the need to disassemble the machine in the case of problems with the optical connection. The risk of pulling out a fibre from its seat is also present during standard printer maintenance. Furthermore the seats also run the risk of accumulating paper residue and other dirt that can have a negative influence on device performance. The technical problem of the present invention is to provide a device for alignment and feed of documents wherein the light transmission between the light sources and the detectors is performed in a reliable manner at reasonable cost . Another problem of the invention, is to obtain a device for alignment and feed of documents that can be reliably tested independently of the equipment for which it is intended, so that it does not need to be assembled with the equipment in order to be tested. The device for paper sheet alignment and feed according to the present invention comprises at least one light source and a corresponding photodetector, arranged on the same side of the feeding plane, at least one light transmitting element, optically connecting said light source and said photodetector, arranged on the opposite side of said light sources and photodetectors with respect to said feeding plane, wherein said sheet is adapted to interrupt the light beam between said light source, said light transmitting element, and said photodetector. Preferably, said transmitting element is associated with a guide plate and comprises pre-shaped optical guides, having ends substantially lying flush with a lower surface of the guide plate. Preferably said ends are positioned within seats of said guide plate, facing said feeding plane at the light source and its corresponding photodetector. In a preferred embodiment, said ends are recessed with respect to said lower surface by a distance ranging between 0 and 2 tenths of a millimetre. In a preferred embodiment, said optical guides functional to alignment are partially encased in a single block. Preferably said alignment optical guides have arms with end sections which protrude from said block and inserted in seats of the guide plate. In a preferred embodiment, said optical guides are moulded together with said guide plate. Preferably said optical guides have ends with a reduced diameter with respect to the other sections of the guides which form a shoulder positioned in contact with a step formed in the seats of the guide plate. Preferably the device of the invention comprises a sealing element adapted to cooperate with said optical guides to block them with respect to said guide plate. In a preferred embodiment, said optical guides are obtained by means of moulding from highly transparent plastic material. The characteristics of the invention will be made clear from the following description provided as a non limiting example and with the aid of the appended drawings. Brief description of the drawings. Figure 1 shows a schematic view in perspective of a device for the alignment and feed of documents, of a known type; Figure 2 shows a diagram of components that operate in combination with the device shown in fig.l; Figure 3 shows a schematic section of a device for the alignment and feed of documents, of a known type,- Figure 4 shows a schematic section of a device for the alignment and feed of documents, according to the invention; Figure 4a is an enlarged detail of the device shown in fig. 4; Figure 5 is a schematic front view in perspective of the device shown in fig. 4; Figure 6 is a schematic rear view in perspective of the device shown in fig. 4; Figures 7 and 8 show an enlarged view of details shown in fig. 6; Figure 9 shows a view in perspective of a variant of the device shown in fig. 4; Figure 10 shows a schematic section of a variant of fig. 4; Figure 10a is an enlarged detail of the device shown in fig. 10; Figures 11 and 12 show a view in perspective of two details of fig. 10; and Figure 13 shows a schematic plan view of the device shown in Fig. 10. Detailed description of a preferred embodiment. Figure 1 shows, with identical numeral identification, the alignment device described in patent USA 5,947,471. This device comprises a set of rollers 15 and counter-rollers 14, a support and feeding plane 10 for a paper document, a hindering bar 16, and motor axles 12 and 13 respectively for the rollers 15 and counter-rollers 14, that are synchronised with each other and set respectively under and above the plane 10. The rollers 15 are keyed on axle 12 and protrude through slots 11 in plane 10 to interfere with the underside of the document. The counter-rollers 14 are composed of pairs of rollers 32 and 33, wherein rollers 32 are keyed on shaft 13 and have a traction surface with a high friction coefficient, while rollers 33 have a larger diameter and a low friction coefficient. The rollers 33 are supported by a hub of the rollers 32 by means of elastic spokes 36, and are friction driven by rollers 32. The bar 16 is positioned downstream of shaft 13, and is normally in a lowered position with respect to the plane 10 in order to arrest the document as it is introduced, and is adapted to be raised together with the start-up of the motor axles 12 and 13. Figures 2 and 3 show in a schematic manner certain components employed in an alignment device, here identified by numeral 44, mounted on a printer manufactured by Olivetti-Tecnost S.p.A. These components contribute towards the detection of the presence of a paper document 46 and its successive alignment, and are connected to appropriate electronic control means not shown in the figures. In device 44, the support and feeding plane is defined by a sliding plate 47 associated with a guide plate 48 positioned above it, and which defines a space 46 for the document movement. The plate 48 is made in plastic with a transversal rib 51 and a edge 49 at the rear part. The plate 48 also includes small shoulders 52 and ribbing 53 of the edge 49 for positioning the shaft 13. The hindering bar identified by numeral 57 is comb-type. It has side appendixes guided by slots in plate 47 and teeth arranged between the pairs of counter-rollers 14 and ribbing 53, positioned tangentially to the rear surface of edge 49. Six pairs of rollers 15 and counter-rollers 14 are envisaged. The detection of the presence of the document is performed by two pairs of leds 59, and photodetectors 61, and by two optical fibres 62, and the alignment is detected by four pairs of leds 63 and photodetectors 64, and by four optical fibres 66. The leds 59 and 63 and the photodetectors 61 and 64 are arranged on the lower half plane of the feeding plane 10, mounted in appropriate openings in the plate 47. The optical fibres are arranged on the upper half plane of plane 10. The optical fibres 62 and 66 are of the commercial type, composed of a transparent plastic core and an opaque sheath. The fibres 62 and 66 are curved as far as their flexibility will permit and their ends are inserted in seats 67 and 68. The seats 67 are formed in small columns protruding vertically from plate 48, while seats 68 are formed in the ribbing 53 from edge 49. The ends of the optical fibres 62 and 66 face the openings 69 and 71 of seats 67 and 68, respectively, aligned with the openings in which the leds 59 and 63, and the photodetectors 61 and 64 are set, and are inserted in the seats 67 and 68 until they abut against a step 65, obtained by the contact of the openings 69 and 71 and the seats 67 and 68, which have a larger width than the openings 69 and 71. The axes of the optical fibres are on substantially vertical planes and the fibres themselves are elastically flexible to permit the raising and extraction of the shaft 13 from the small shoulders 52, when the counter-rollers 14 require maintenance. According to the present invention, it has been observed that the distance between the step 65 and the lower surface of the plate 48, typically of about one or more millimetres, can accumulate scraps, dust or similar substances, such as to interfere with the correct reception of the signal emitted by the leds 59, 63. In order to detect the presence of the document, the two pairs of leds 59 and photodetectors 61 are aligned along a line parallel with the axes of the shafts 12 and 13, slightly upstream of the rollers 15. The four pairs of leds 63 and photodetectors 64 involved with the alignment of the document are arranged in line on the plate 47, slightly downstream of the rollers 15, and adjacent to the feeding plane of bar 57. Moreover, the pairs of leds 63 and photodetectors 64 are interlaced with each other in an alternating mode. The introduction of a document 46, at whatever angle, in the front part of the device 44 is detected by the photodetectors 61. In fact the led 59 emit in a direction perpendicular to the plane 10, light beams 72, which are re-transmitted through the openings 69 and the optical fibres 62 to the photodetectors 61. The document 46 introduced between the plates 47 and 48, interrupts the beams 72 blacking out one or both photodetectors 61, thus activating the relative control means. This triggers the rotation of the rollers 15 and counter-rollers 14, causing the displacement of the document and its alignment against the bar 57. In fact the low friction coefficient rollers 33 cause the entering edge of the document to undergo a progressive alignment against the bar 57. In a similar manner as for the detection of the presence, the edge of the document 46 in alignment interrupts the beams 73 emitted by one or more leds 63 causing the blackout of one or more photodetectors 64. When all photodetectors 64 have been blacked out by the document already aligned, the relative control means trigger the raising of the bar 57 by mechanical means, also not shown in the figures. At the same time, another mechanism exercises pressure on the shaft 13 in the direction 74 indicated by the arrow, moving the high friction coefficient rollers 32 against the document 46. Thus, the document is pushed with a high adherence level and perfectly in alignment towards the rear part inside the printer. The alignment device according to the present invention is represented by numeral 81 in figures 4, 5 and 6, in which the parts having the same functions as those of device 44 maintain the same numerals. In device 81, the presence detecting leds for the document are identified with numerals 59-1 and 59-2, and the relative photodetectors are identified with numerals 61-1 and 61-2. The alignment leds are identified with numerals 63-1, 63-2, 63-3 and 63-4 and the relative photodetectors are identified with numerals 64-1, 64-2, 64- 3 and 64-4. The beams 72 emitted by the led 59 and intended for the photodetectors 61 to detect the presence of the document 46, are transmitted by optical guides 82. In turn the optical guides 83 transmit the beams 73 emitted by the leds 63, intended for the photodetectors 64 and used to align the document. The guides 82 and 83 are made of highly transparent plastic material and the ends thereof are positioned in front of the relative leds and photodetectors. According to the invention the optical guides 82 and 83 are firmly attached to the guide plate 48 and are pre-shaped so that they embrace the counter-rollers 14 and permit easy removal of shaft 13. As an example, the optical guides 82 and 83 have a quadrangular section and are realised in moulded polycarbonate. The guides 82 and 83 have a basically horseshoe shape, and a bridge shape respectively, and when in use they are arranged around the six pairs of rollers 32 and 33 : the two guides 82 in a horizontal direction and the four guides 83 in a substantially vertical direction. In detail, the optical guides 82 (Figures 5 and 7) are identical to each other and each have two arms 91a and 91b connected by a section 92, and have two ends 93a and 93b folded over in a downward direction. The guides 82 are arranged parallel with the plate 48 between rib 51 and shoulders 52. The ends 93a and 93b are inserted into seats 84 of the plate 48 (Figure 5) in front of the leds 59-1 and 59-2, and the photodetectors 61-1 and 61-2. The arms 91a and 91b extend in the direction 21 and the sections 92 are parallel with the shaft 13. Observing the device 81 from the front part in direction 21, in the guide 82 that is conventionally the first guide, the end 93a is set on the left hand side of the second pair of rollers 32 and 33, and the other end 93b is set on the left hand side of the third pair of rollers. In the second guide 82, the end 93a is set on the right hand side of the fourth pair of rollers 32 and 33, and the end 93b is set on the right hand side of the fifth pair of rollers. The guides 82 are anchored to the plate 48, by means of gluing or insertion using wedges 94 that act on the sections 92. In this manner the guides 82 are under the shaft 13, they surround the pairs of counter-rollers 14 and do not interfere with the possible trajectory of the shaft 13 in the case of removal . The optical guides 83 (Figures 6 and 8) too are identical to each other and each have two arms, 96a and 96b connected by means of curved portions by a section 97, and ends 98a and 98b. The guides 83 are arranged downstream of the shaft 13 and are developed in a vertical direction with respect to plate 48. The arms 96 are inserted in seats 99 formed in the ribbing 53 in a manner so that the ends 98a and 98b are facing the underlying leds 63 and photodetectors 64. In a suitable manner, the guides 83 (also see fig. 9) are encased largely in a shaped resin block 101. The block 101 is positioned over the pairs of counter-rollers 14, and at each pair, it has spaces 102 and recesses 103 to permit the rotation of the counter-rollers and to facilitate the extraction of shaft 13 in case of maintenance. In detail, block 101 totally encases the horizontal connecting section 97, and partially, the two arms 96-a and 96-b of each optical fibre 83 at projections 104 between the recesses 103. The lower sections of the arms 96-a and 96-b of each guide 83 stick out under the projections 104 of the block 101 and are inserted in the respective seats 99 of the ribbing 53. The block 101 is firmly attached to the edge 49 by joint or gluing between its lower edge and the upper edge of the edge 49. The solution involving the total encasing in a single block eliminates the need for interventions following defects in optical fibre insertion and consequential errors, without hindering any possible maintenance on shaft 13 and on pairs of rollers 32 and 33. Moreover, the device 81 can be completely tested before it is assembled on the intended equipment without its characteristics becoming adversely affected by said assembly operations. In detail, each single guide 83 is positioned to embrace the single pairs of rollers 14 in an interlaced manner. With reference to figure 6, the first optical guide 83 is developed from the axis of led 63-1, with the arm 96- a on the right hand side of the first pair of rollers 32 and 33, then it bends to the left for the horizontal length 97, and then descends with arm 96-b in front of the photodetector 64-1, on the left hand side of the second pair of rollers. The second guide 83 starts from the axis of led 63-2, rises with arm 96-a on the right hand side of the second pair of rollers 32 and 33, then bends to the left for the horizontal length 97, to then descend with arm 96-b in front of photodetector 64-2, on the left hand side of the third pair of rollers. The third optical guide 83 follows a symmetrical pattern with respect to that of the second guide 83 : it starts from led 63-3, rises on the right hand side of the fourth pair of rollers, bends to the left, and descends in front of the photodetector 64-3 on the left hand side of the fifth pair of rollers. Lastly, the fourth guide 83 follows a symmetrical pattern with respect to that of the first guide 83: it starts from led 63-4, with arm 96-a on the right hand side of the fifth pair of rollers, bends to the left for the horizontal length 97, then descends with arm 96-b in front of the photodetector 64-4 on the left hand side of the sixth pair of rollers 32 and 33. According to the invention, the optical guides 82 and 83 are inserted in the seats 84 and 99 so as to completely close them, to prevent any paper residue or dust from entering the openings, thus preventing device malfunction. In particular, the guides 82 are anchored to the plate 48 by gluing or joint using wedges 94, while the guides 83 are encased in block 101, which is in turn attached to the edge 49 by joint or gluing. In this manner it is possible to eliminate the step 65 (fig. 3) against which the optical guides abut in the device according to the known art, permitting ends 93a and 93b of guides 82 to be completely inserted in the seats 84 of the plate 48 (Fig. 4a) in a manner so that they can be closed for their total depth. The ends 93a and 93b are substantially flush with the lower surface 41 of the guide plate 48, which defines, with the sliding plate 47 the space for the movement of the document 46, and has not protrude from the plate in order to prevent interference with the movement of document 46. The term 'substantially flush with' means that the ends 93a and 93b must be recessed with respect to the surface 41 by a distance D, which may range for example between 0 and 2 tenths of a millimetre (Fig. 4a) . The distance D is sufficiently reduced to prevent accumulation of scraps, dust or the like that could interfere with the correct optical functioning of the device. In a similar manner the ends 98a and 98b of the guides 83 are positioned substantially flush with the lower surface 42 of the ribbing 53, providing the advantage that the ends 98a and 98b can be realised in a manner to follow the contour of the surface 42, which is slanted with respect to that of the guide plate 48, so as to facilitate the introduction of the paper. The guide plate 48 is made of an opaque resistant plastic material. In a variant of the present invention, the optical guides 82 and 83, having different characteristics from those of plate 48, are co- moulded with the plate 48 itself. In another variant 105 (Figures 10-13) of the device according to the invention the two types of optical guides for document presence and alignment, identified by numerals 106 and 107, maintain the realisation in pre- shaped highly transparent plastic material, but they have a substantially circular section and are both set in a vertical direction with respect to the plate 48. In detail, the optical guides 106 upstream of the shaft 13, are horse-shoe shaped with two arms 108 and a curved connecting section 109. The arms 108 each have a terminal vertical section and a curved section, slanted with respect to the vertical direction and connected by section 109. The vertical sections of the arms 108 are inserted in seats 111 formed in small columns perpendicularly projecting from the plate 48, while the section 109, has a concave surface facing upwards and is slanted in the direction of the shaft 13. The optical guides 107 have a bridge-shaped configuration, with two substantially vertical arms 112 connected by curved portions and a horizontal section 113. When in use the guides 107 embrace the pairs of counter- rollers 14 in an interlacing manner, and the arms 112 are positioned inside seats 114 formed in the ribbing 53 of edge 49. In order to maintain the optical guides 106 and 107 correctly inserted in the seats 111 and 114, avoiding the problem of incorrect positioning, a substantial sealing element is used, such as for example, a casing 116. The casing 116 (figures 10 and 13) is in plastic material and defines two conventionally rear and front sections 117 and 118. The section 117 comprises an end wall 119 and an intermediate wall 121 connected at the top by a concave surface 122, perpendicular to plate 48 when in use. The section 117 houses the connecting sections 113 and part of the arms 112 of guides 107. The surface 122 is positioned over the counter-rollers 14 while the lower edges of the walls 119 and 121 rest on the upper part of the edge 49. The walls 119 and 121 are notched to follow the profile represented by the counter-rollers 14. The front section 118 defines two projections 123, a flange 124, and four small hollow columns 126. The projections 123 are "dome-shaped" and house the second and fifth pair of counter-rollers 14 and part of the optical guides 107. In the flange 124, between the projections 123, are two slots 127 in which the third and fourth pairs of rollers 14 can rotate. The small columns 126 protrude from the flange 124, and can house the screws 128, and they are adapted to rest on plate 48. During assembly, the casing 116 elastically deforms the guides 106 and 107 when approaching. In detail, the slanting surfaces of the projections 123 deform the slanting sections of the arms 108, and the walls 119 and 121 define the exact positions of the sections 107 and the arms 112. When the small columns 126 are in contact with the plate 48, the concave surface 122 exercises an opposing and sealing action on the four guides 107, preventing any rising from seats 114. At the same time, the upper parts of the "dome-shaped" projections 123 exercise an opposing action on the sections 109 of the guides 106 preventing the guides 106 from rising from seats 111. The casing 116 can be removeably attached to the guide plate 48, in a blocked condition by means of the screws 128 of the small columns 126 screwed into the plate 48. In device 105 of the invention, the optical guides 106 and 107 are inserted in the seats 111 and 114 and are realised in a manner so that they completely close the openings 69 and 71 to prevent any paper residue or dust from entering the openings, and to prevent device malfunction (Fig. 10a'.) In particular, the guides 106 and 107 have ends 120a, 120b, 121a and 121b, with a diameter that is smaller than those of the other sections 108, 109, 112, and 113 of the guides, and such as to permit the insertion of the ends in the openings 69 and 71, and a shoulder 165 at the beginning of the guide sections having a different diameter (Figures 11 and 12) . When the casing 116 is blocked on plate 48, the shoulder 165 is placed in contact with step 65 obtained by the contact between the openings 69 and 71 and the seats 111 and 114, and the ends 120a, 120b, 121a and 121b have a length that permits them to be positioned substantially flush with the lower surface 41 of the guide plate 48, that defines, together with the sliding plate 47, the space for document 46 movement. Also in this case, the ends 120 and 121 should not protrude from the plate so that they do not interfere with the movement of document 46. The term 'substantially flush with' means that the ends 120a, and 120b are recessed, with respect to the surface 41, by a distance D which can preferably range between 0 and 2 tenths of a millimetre (Fig.10a) . Similarly, the ends 121a and 121b of the guides 107 are substantially flush with the lower surface 42 of the ribbing 53, and this provides the advantage of being able to realise the ends 121a and 121b in a manner to follow the contour of the surface 42, that is slanted with respect to that of the guide plate 48, in order to facilitate the insertion of the paper.