Add publishing condition that waits for sky maps before sending an alert

As per request of the RAVEN review committee, we should add a condition where we wait to send an alert for any GW-GRB coincidences until we have both sky maps available (and have calculated a joint FAR with sky map info) as we currently do with Swift coincidences. This will add some latency but will reduce false alarms.

Note that this requires #547 (closed) to be fixed prior to implement this.