docs: def get_filenames(dirname):
      """Return all model output filenames inside a model output directory,
      sorted by iteration number.
  
      Parameters
      ----------
      dirname: str
          A path to a directory.
  
      Returns
      -------
      filenames: list[str]
          Paths to all output files inside `dirname`, sorted in order of
          increasing iteration number.
      """
      filenames = glob.glob('{}/*.pkl'.format(dirname))
      return sorted(filenames, key=_f_to_i)