Incorporate GW-FRB search with CHIME

Since CHIME VOEvents are now available, we will want to be able to incorporate a GW-FRB search into RAVEN. This requires the following:

• Determine whether the [-2,+12]s search window used elsewhere in the FRB searches (PyGRB and X-pipeline) should be used here as well
• Calculate the approximate number of CHIME events per year
• Determine how sky map information from CHIME can be incorporated
• Ask Alex Pace to add CHIME and FRB fields to GraceDB (pipeline and search values; ingestion of FRB VOEvent)

A related issue is #20.