One line: another proposed fix to Bug 29

I had originally made the interface for cvmix_compute_enhanced_diff() fairly complex because it needed to write to OBL_[MTS]diff if the OBL depth was above the cell center of the cell containing it and write to [MTS]diff in cases where the OBL depth was below the cell center of the cell containing it. If ktup = kwup then OBL_[MTS]diff shouldn't be accessed, so we can pass in OBL_[MTS]diff(kwup) rather than padding the array.

Also, this lets us remove the select case call from the routine and replace it with something a little more readable (I hope).