Include `NOT_GRB` label when deciding to update preferred external event

The current logic of raven.update_coinc_far could lead to a situation where the preferred GRB has the NOT_GRB label due to a classification or technical issue, and should not be preferred over a neighboring GRB candidate. We could consider including logic that doesn't allow a NOT_GRB to override an existing preferred event and rerun this check if the label is removed.