Add ``--work-spec`` to an :mod:`argparse` `parser`.

def _add_work_spec_args(self, parser):
        '''Add ``--work-spec`` to an :mod:`argparse` `parser`.'''
        parser.add_argument('-w', '--work-spec', dest='work_spec_path',
                            metavar='FILE', type=existing_path,
                            required=True,
                            help='path to a YAML or JSON file')