Draft: Add HasMassGapp probability to VO events

We are including the HasMassGapp probability in O4 Notice. Here there is a modification of the code to allow the additional fields

assigned to @roberto.depietri

mentioned in issue computing/gracedb/server#243 (closed)

Related issue: computing/gracedb/server#243 (closed)

@roberto.depietri I'm assuming this is supposed to be merged into upstream's master branch. You have it set to merge into your fork's master branch, however. I can't modify this, but you should be able to.

It was implemented in gracedb in another merge request. Closing

closed