docs: def out_name(stem, timestep=None):
      """Return StagPy out file name.
  
      Args:
          stem (str): short description of file content.
          timestep (int): timestep if relevant.
  
      Returns:
          str: the output file name.
  
      Other Parameters:
          conf.core.outname (str): the generic name stem, defaults to
              ``'stagpy'``.
      """
      if timestep is not None:
          stem = (stem + INT_FMT).format(timestep)
      return conf.core.outname + '_' + stem