Automatically determine whether sky map is MOC or flattened

This can be done similarly to emfollow/gwcelery!1495 (merged)

def is_skymap_moc(skymap_bytes):
    with NamedTemporaryFile(content=skymap_bytes) as skymap_file:
        # If ordering is EXPLICIT, should be multiordered (MOC)
        return getheader(skymap_file.name, ext=1)['INDXSCHM'] == 'EXPLICIT'

Also checks whether the input is an astropy table or 1-D array could work for skymap_overlap_integral rather supply the UNIQ columns separately.