returns a list of all files with the given filename pattern in the
        given PCC annotation layer

def get_files_by_layer(self, layer_name, file_pattern='*'):
        """
        returns a list of all files with the given filename pattern in the
        given PCC annotation layer
        """
        layer_path = os.path.join(self.path, layer_name)
        return list(dg.find_files(layer_path, file_pattern))