Ignore known Fermi and Swift noise triggers in RAVEN

Fermi sends us classification on the likely source for their GRB candidates. We can flag many of the candidates that are very likely noise (<50% chance of being astrophysical) with the DQV label and not consider them initially for the RAVEN analysis (note: we chose not to filter them out completely because classifications can change in the future with additional GCN notices from Fermi.)

The design is as follows:

• If a GCN event comes in where the most likely source is not a GRB or this classification is less than 50%, we flag this external event with label DQV.
• When considering coincidences for release, RAVEN checks for the DQV label and will not send a preliminary notice if this label is present on the external event.
• Events can have DQV removed or added depending on whether future updates to it pass or fail this classification test.

To do:

• Add classification check to external_triggers.py
• Add label check to raven.py
• Make tests