returns a list of all files (from all available annotation layers)
        with the given document ID.

def get_files_by_document_id(self, document_id):
        """
        returns a list of all files (from all available annotation layers)
        with the given 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)))