Fix threshold to apply 'NOT_GRB' label to GRBs

The label 'NOT_GRB' seems to being overapplied due to the fact that notices are now using percentages for probability rather than a scale of 0-1. This is causing events to get this label applied when not applipical. See the example of E388726 where the initial_data,0 has values of 'Most_Likely_Prob' from 0-100, misapplying the label which was later removed. This percentage form is corroborated in the GCN notice https://gcn.gsfc.nasa.gov/other/657554740.fermi.

This could be fixed by change the value here to 50 instead of .5 .