Limit GFS forecasts to 12 hours in the future

Matches original aitf-mirror behavior in cache update mode.

example/aitf/ancil.py

@@ -103,14 +103,8 @@ class GFSDownloader(csppfetch.Downloader):
         # We can use forecasts from this far into the future
         self.shortest_valid_forecast = dt.timedelta(hours=3)
         # ...through this far into the future.
-        self.longest_valid_forecast = dt.timedelta(hours=15)
-        # Given that, valid forecasts are 3, 6, 9, 12, and 15.
-
-        # (The original specification allowed up to 12 hours,
-        # but was only tested against the start time of each pair,
-        # allowing the 15 forecast to sneak in. We want that
-        # because otherwise at 0:00-2:59 and every 6 hours after,
-        # there would not be a valid fallback option.)
+        self.longest_valid_forecast = dt.timedelta(hours=12)
+        # Given that, valid forecasts are 3, 6, 9, and 12.
 
         valid_multiple = (self.shortest_valid_forecast-self.first_forecast)/self.forecast_step
         if not valid_multiple.is_integer():