docs: def write_options_to_file(self, filename, file_format='yaml'):
          """Write options to file.
  
          This is a wrapper over :func:`.write_options_to_JSON` and
          :func:`.write_options_to_YAML`.
  
          :param str filename: Target file to write the options.
          :param str file_format: File format (``yaml`` or ``json``).
  
          :raises:
              :ValueError: If an unknown ``format`` is requested.
          """
          if file_format.lower() == 'yaml':
              self.write_options_to_YAML(filename)
          elif file_format.lower() == 'json':
              self.write_options_to_JSON(filename)
          else:
              raise ValueError('Unknown format {}'.format(file_format))