Ensure `raven_calc_signif_gracedb` uses the correct method based on sky map filenames

There is a minor bug where currently we are calculating the coincidence FAR with the assumption that the GW sky map can only be multiordered: https://git.ligo.org/emfollow/gwcelery/-/blob/b8322e10d5023f217b71a3ccaf90dd92efe23210/gwcelery/tasks/raven.py#L51

We should add a statement checking that the word multiorder is in the GW sky map name to use the se_moc method, and set to false if not. We could also do something similar to the external sky map name as well, although this is less pressing.