The present invention relates to a method and a processor for controlling the spatial position of a direct digital x-ray detector in a medical imaging system comprising multiple radiographic exposure stands. More in particular it relates to a method and processor for controlling the spatial position of a digital direct x-ray detector intended for use in a planned radiographic exposure. Said control includes checking whether the detector is correctly positioned within the radiographic exposure stand of the medical imaging system. Background of the invention. It is known that radiographic illumination or exposure has important applications in medical imaging, whereby the medical advantages for the patient largely exceed the small risk of damage resulting from such radiographic illumination. In earlier days radiographic exposures mostly made use of film based on silver halide technology as image capturing medium. Since a number of years the so-called computed radiography technique has gained wide market acceptance. This technology makes use of a radiographic panel that does not use silver halide technology as the light capturing medium, but uses stimulable phosphors. This method is described amongst others in detail in the Handbook of Medical Imaging, (ed. R.V. Matter et al., SPIE Press, Bellingham, 2000). During recent years, radiographic exposures increasingly make use of direct digital radiographic techniques, known as DR (Direct Radiography). This method is increasingly used as alternative for film-based imaging techniques, as well as for the panels based on the use of stimulable phosphor-technologies, as described supra. In this digital radiographic method the radiographic exposure energy is captured pixelwise in a radiographycally sensitive panel, and hereupon is converted to electronic image data by means of electronic components. Hereupon the information is read out imagewise and displayed on a suitable monitor for diagnostic purposes by a radiologist. One of the driving forces behind the success of direct digital radiography is the ability to rapidly visualise the radiographic images and to efficiently and simply communicate over datanetworks to one or more sites for analysis and remote diagnosis by a radiologist or other medical expert. The delays that are characteristic for the development, packaging and physical transport of radiographic films are avoided by the above methods. Also the difficulties arising from the scanning of developed films and the corresponding loss in resolution is avoided by the above techniques. The advantage of direct radiographic systems over computed radiographic systems, based on stimulable phosphors, is that no read-out (in a digitizer) of the latently captured radiographic image needs to take place. On the contrary, the digital radiographic image promptly or directly can be read for the purpose of evaluating the image from a diagnostic point of view. This diagnosis can take place at a local or remote workstation. At the beginning the first direct radiographic panels were integrated in the overall radiographic imaging system. The wiring was designed such that minimal trouble to the radiographic operator was caused hereby when the radiographic direct panel was placed for exposure of a body part of a patient. More recently portable direct radiographic panels have been introduced to the market place. These panels make use of an on-board battery and communicate with the radiographic control panel or workstation, as well as with the datacapturing apparatus and the display components in a wireless manner. The latter aspects resulted in a wide acceptance of such portable wireless panels by the marketplace and ensures their practical use in a fully digital radiographic exposure system. In a hospital or medical diagnosis center, these panels can be used as well in a completely newly installed radiographic imaging system or in a so-called retrofit situation. The term retrofit should be understood as directed to an existing radiographic system, that previously made use of radiographic films or stimulable phosphor plates, and whereby the latter registration media have been replaced by a direct radiographic capturing medium, a so-called direct radiographic or DR panel, without the need to replace the workstation or the radiographic source itself. The advantage of such a retrofit radiographic system as compared to a completely newly installed radiographic system, is its lower investment cost, as part of the already installed radiographic system can be re-used. Although portability and wireless communication of the radiographic registration medium clearly is an advantage when portable and wireless DR panels are used, these features also are characterized by the occurrence of problems under practical circumstances of use. In particular such panels are characterized by identification, or position difficulties when they are used in a so-called multi-modality environment. This may lead to mistakes for example when exposing the wrong detector or panel, or exposing a mis-positioned panel. Contrary to radiographic films or stimulable phosphor panels that after exposure need to be removed from the radiographic exposure room for the purpose of being developed, resp. for being read-out in a digitizer, direct radiographic panels after use can remain in the radiographic exposure room. When as a result of the above situation various direct radiographic panels are available in the radiographic exposure room, the radiographic operator needs to be fully sure that for the next or planned radiographic exposure the right panel needs to be identified or selected and that this panel is correctly positioned in the correct exposure modality. Absent same it would be possible to expose the wrong DR Panel, or to reset same, or the collect the data hereof, or to expose an entirely or partly ill-positioned detector. Without a specific method that enables to reduce to an absolute minimum the probability of mis-positioning an x-ray Detector, there remains an enhanced risk for an incorrect exposure of a patient, resulting in retakes. On its turn, this results in a number of complaints, confusion, and a loss of time and efforts. In US Patent Application US 2011/0305319 Al, published December 15, 2011, in the name of General Electric Company, NY, USA, a portable x-ray detector and a gravity sensor coupled thereto is described. Aprocessor is coupled to the gravity sensor, programmed to receive an input from the gravity sensor, determine a physical orientation of the portable x-ray detector based on the received input, and generate an indication to reposition the portable x-ray detector. The aim of such gravity sensor and coupled processor is to solve the problem when the operator positions the x-ray detector out of alignment with respect to the x-ray source. Apart from the above, this specification discloses no other function associated with such gravity sensor and its coupled processor. Problem to be solved The method as described above may well solve the problem of a correctly positioned x-ray detector but that is out-of alignment with respect to the corresponding x-ray source. The issue of ensuring that the correct x-ray detector is selected in a multi-panel environment, and that such selected x-ray detector is positioned in the correct exposure stand of the medical imaging system wherein the planned radiographic exposure is planned, is not addressed in the above specification. Nor is it addressed there that even if such detector is placed in the correct exposure stand, that is is in spatial alignment with the radiographic source in said stand. As a result hereof there remains a need for an easy and convenient method for ensuring that in a planned radiographic exposure the selected x-ray detector is correctly positioned in the exposure stand of the medical imaging system, before any such radiographic exposure takes place. The aim and purpose of the invention is to avoid the abovementioned problems by providing such an effective method and corresponding processor. Summary of the invention. The abovementioned aspects are realised by means of the processor and the method as described in the independent claims set forth hereinafter. Specific features of preferred embodiments of the invention are set forth in the dependent claims. Further advantages and embodiments of the present invention are clarified in the description that follows. Description of the invention : According to the present invention, a method is provided for controlling for a planned radiographic exposure the spatial position of a portable digital direct xray detector in a medical imaging system comprising multiple radiographic exposure stands , said method comprising the following steps : ° creating a threedimensional spatial model of the medical imaging system ; ° determining within said model the spatial position of said detector based on input received from three gravity sensors installed within said detector; ° determining within said model a three-dimensional volume space for each radiographic exposure stand comprised in said medical imaging system; ° checking whether for the planned radiographic exposure the spatial position of said detector to be used in said exposure fits within the three-dimensional volume space of the radiographic exposure stand to be used in said exposure. According to a preferred embodiment of such method, the threedimensional spatial model of the medical imaging system comprises reference spatial positions for said detector. According to a further preferred embodiment, said reference spatial positions for the detector relate to one or more of the following stands : wall, table, storage and/or docking position. According to a further preferred embodiment, the spatial position of said detector is determined on the movement of said detector relative to said reference points by double integration of acceleration values as measured by accelerometer sensors comprised in the gravity sensors installed on said detector. Further according to the invention, a processor is provided for controlling for a planned radiographic exposure the spatial position of a portable digital direct xray detector in a medical imaging system comprising multiple radiographic exposure stands, said processor comprising the following means : ° means for creating a threedimensional spatial model of the medical imaging system; ° means for determining within said model the spatial position of said detector based on input received from three gravity sensors installed within said detector; ° means for determining within said model a three-dimensional volume space for each radiographic exposure stand comprised in said medical imaging system; ° means for checking whether for the planned radiographic exposure the spatial position of said detector to be used in said exposure fits within the threedimensional volume space of the radiographic exposure stand to be used in said exposure. According to a preferred embodiment, the threedimensional spatial model of the medical imaging system comprises reference spatial positions for said detector. Said reference spatial positions for the detector may relate to one or more of the following stands : wall, table, storage and/or docking position. According to a further preferred embodiment, the gravity sensors installed within said detector comprise accelerometer sensors, and the spatial position of said detector is determined on the movement of the detector relative to said reference points by double integration of acceleration values as measured by said accelerometer sensors. According to a further preferred embodiment, apart from the accelerometers for determining the spatial position of the detector, said detector also comprises a gyroscope to determine the orientation of the detector. In a further preferred embodiment of the invention the above processor is part of the radiographic work station of a medical imaging system. According to a still further preferred embodiment, the medical imaging system comprises an x-ray source provided with means for determining its spacial position, said position being communicated to the radiographic workstation. Tracking means can then be provided enabling the (re-)positioning of this x-ray source to be in alignment with the detector. According to a preferred embodiment, the x-ray detector comprises one or more accelerometers as gravity sensors. To determine the translation of an object, such as an x-ray detector, three onedimensional accelerometers, or one three-dimensional (3G-) accelerometer is preferably used. So as to obtain spatial or positional information from accelerometer sensors, the measured values of acceleration (in 1, 2 or 3 axes) preferably are integrated twice. Asingle integration will lead to the velocity (in all 3 axes), a subsequent integration of the velocity will yield the positional information (in all 3 axes). Care should be taken to rule out earth's gravity effect. In the formulae below, a denotes acceleration, v denotes velocity, t denotes time and s position. a = dv/dt and v = ds/dt so a = d 2s/d 2t thus s= v dt = ( a dt) dt Asimple double integration of such acceleration values will only provide relative position or differences in position in all 3 axes. As not only the relative position, but also the absolute position of a detector should be determined, the reference position of the detector must be determined prior to the motion of the detector. As an example illustrative for the operation of the present invention, an x-ray detector may be equipped with 3 accelerometers, 1 for each axis of the detector x, y and z . Prior to operation of a planned radiographic exposure, the x-ray detector is placed in its 'home' position and remains motionless. Then a calibration is performed which leads to initial position values xO, yOand zO. This home position can be a predetermined position within the medical imaging system. Positions of interest in the radiology unit (DR modality positions like table and wallstand) must be determined also. This usually is performed at the time of initial installation of the medical imaging system. Such positions of interest are then calculated as xl