DICOM hierarchy simplifications

This release contains small changes with a big impact. We removed the ImageFile entity (connected to each DICOM Image entity) and no longer persist the file name and source of each image. The filename is now derived directly from each Image.

Further, we changed how uniqueness is defined for Study:s, Series and Images. Before, these were defined using StudyInstanceUID, SeriesInstanceUID and SOPInstanceUID respectively. This could (and sometimes did) lead to inconsistencies in the DICOM hierarchy when adding a new file, for instance when a new file lead to adding a new Patient to the system, but the Study was already added from before and connected to another Patient. We now define uniqueness as the xxInstanceUID properties paired with the parent id. For instance, this means that a Series is defined by its SeriesInstanceUID paired with its study id.