One line: another proposed fix to Bug 29

Details: I had originally made the interface for cvmix_compute_enhanced_diff()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).
Merge pull request #31 from CVMix/bugfix/out_of_bounds_OBL_Mdiff-option2

Fix for bug 29