Add RAVEN fields to VOEvent
Description of feature request
RAVEN is being approved to send alerts, so we need to add some additional fields to the existing VOEvent to properly communicate with astronomers. These fields are described in emfollow/userguide!65 (closed). There are seven additional fields.External GCN Notice ID, External Alert Type, and External Search are essential and should be required. Time and Sky Position Coincidence FAR, Combined Sky Map, and Time Difference are optional and should be included only if the values are available. Time Coincidence FAR needs to be included if the search field of the external event is GRB or SubGRB, but not if the search is SNEWS since that value won't be calculated in that case. If that isn't possible then Time Coincidence FAR could be made optional in general. Note that if Time and Sky Position Coincidence FAR is available, Time Coincidence FAR will alway be available as well.
I can't see any default values that could be set that wouldn't cause confusion.
Use cases
This template should be used whenever we are creating a VOEvent and EM_COINC is present in the superevent's labels.