Normalize longitude ranges

[rabernat]

Some datasets use 0-360 longitudes, while others use -180-180. I've dug around, and there doesn't appear to be a universal standard that says one or the other is "right" (e.g. cf-convention/cf-conventions#435)

Currently, EDR users have to know which convention their dataset employs and encode their longitudes accordingly.

It would be more graceful if we could automatically do this on their behalf, automatically translating longitude into the proper range for a given dataset.