return a normalized version of abmn

def _normalize_abmn(abmn):
    """return a normalized version of abmn
    """
    abmn_2d = np.atleast_2d(abmn)
    abmn_normalized = np.hstack((
        np.sort(abmn_2d[:, 0:2], axis=1),
        np.sort(abmn_2d[:, 2:4], axis=1),
    ))
    return abmn_normalized