Dangerous definition of GCN rates

The GCN rates are currently defined as

gcn_rate = 547 / (365 * 24 * 60 * 60)

This code evaluates to zero in Python 2. I am sure RAVEN runs under Python 3 already and plans to do so in the future, but let's protect ourselves from the occasional crazy situation. I recommend making the various constants floats by using trailing periods.