def add_angle_unit_arg(parser):
    """
    Add angle unit selectiona argument to parser.

    Is defined by ``-a`` and ``--aunit``.

    Wether angles are to be calculated in degrees or radians.

    Parameters
    ----------
    parser : `argparse.ArgumentParser <https://docs.python.org/3/library/argparse.html#argparse.ArgumentParser>`_
        The argument parser to add the topology positionl argument.
    """
    parser.add_argument('-a', '--aunit', help=
        'Angular unit, either degrees or radians.', choices=['degrees',
        'radians'], default='degrees')
