docs: def filename(self, fname, timestep=None, suffix='', force_legacy=False):
          """Return name of StagYY output file.
  
          Args:
              fname (str): name stem.
              timestep (int): snapshot number, set to None if this is not
                  relevant.
              suffix (str): optional suffix of file name.
              force_legacy (bool): force returning the legacy output path.
          Returns:
              :class:`pathlib.Path`: the path of the output file constructed
              with the provided segments.
          """
          if timestep is not None:
              fname += '{:05d}'.format(timestep)
          fname += suffix
          if not force_legacy and self.hdf5:
              fpath = self.hdf5 / fname
          else:
              fpath = self.par['ioin']['output_file_stem'] + '_' + fname
              fpath = self.path / fpath
          return fpath