code: def get_files_by_document_id(self, document_id):
          
          assert isinstance(document_id, basestring), \
              "The document ID must be given as a string, e.g. 'maz-1423'"
          return list(dg.find_files(self._all_files, '*{}.*'.format(document_id)))
docs: returns a list of all files (from all available annotation layers)
          with the given document ID.