Add workflow for multiordered GRB sky maps; fixes #581

This is a fork of naresh.adhikari/gwcelery:grb-moc-skymap-workflow and !1287 (closed)

• Allow get_skymap_filename to find a multiordered sky map for external events
• Indicate external sky map is moc when appropriate
• Create/modify function to combine sky maps to also use a multiordered GRB sky map
• Create multiordered GRB sky maps by modifying our current tools
• Enable Swift to calculate joint FAR in the standard method and create combined sky maps with