loadFile:  Fix for 2D case of _MappedGeoGridFlatField binary override -
leverage "make2DGridFromSlice" functionality in GridMath when needed.

Note, for now this only fixes the "a-b" syntax, not the "sub(a, b)"
syntax.  If this fix passes testing, overriding sub, add, etc. to take
advantage of this fix should be straightforward.

[1589]