Issue 628 regridder regird2 mthod gens nan

@williams13 @aashish24 @dlonie @ThomasMaxwell this actually fixes a nasty bug due to the new numpy (missing value is defaulted to nan) that led to erroneous result for regrid2 (at least) @durack1 as been testing this for a few days and while he confirms this fixes his issue with the regridder he didn't see any nasty side effects.

I think this should be merged as soon as possible.