Made the relationship between the book, the book_tei_representation and it's file visible, and made it possible
to create the whole thing with a single 'create book'.
The actual file for the 'BasicFile' of the TEI representation is not loaded correctly. Seems to be passed on as a wrong format (a string instead of the ActionDispatch::Http:UploadedFile required).
Very few unit-tests have been made. More should be created.