LatLon grid of full NCEP GFS advertises the wrong left longitude

[bartdevriendt]

When reading a full domain GFS file the Lo1 of the LatLonGridDefintion is shown as 180. Lo2 is equal to 179.75. This makes querying the data inconsistent.

The fix is to check if Lo1 > Lo2 and subtract 360 from Lo1 if that's the case.