Regional reductions should raise if the region isn't fully defined on the given array

[spencerahill]

Similar but different to #229. If the array upon which regional reductions are being performed does not span the entire globe, then it is possible that some Regions will be either partially or entirely missing from its span.

In either case, the regional reduction is ill-defined and thus should raise an error, or at least raise a warning and return NaN. Currently, if the region is entirely outside the domain, it does return NaN, but if it is only partially so, then it gets computed incorrectly as if the part within the domain was the whole region.