def out_name(stem, timestep=None):
if timestep is not None:<EOL><INDENT>stem = (stem + INT_FMT).format(timestep)<EOL><DEDENT>return conf.core.outname + '<STR_LIT:_>' + stem<EOL>
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'``.