Add workflow for multiordered GRB sky maps

During a joint LVK-Fermi-Swift call, it was brought up that HEALPix multiordered IPN skymaps will be available for O4: http://www.ioffe.ru/LEA/GRBs/GRB230307_T56646/IPN/

However we can't currently ingest these as our workflow assumes the GRB sky map is flattened and we require modifying or adding new functions to enable this:

• 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