#599 - bug fix for calling LookupTable on non-square arrays

This is a tiny pull request for the bug that @arunkannawadi found when calling LookupTables on arrays that are 2d but not square.

I also put this case into the unit tests in case any future changes to this class might break that functionality.

This was small enough that I wasn't even sure it needed a PR, but just in case, here it is.
Good point.  Fixed: https://github.com/GalSim-developers/GalSim/commit/29d76a45ae71cc4ec4c8527ad932bb8c29a16a5e
(I only did the non-square case.  I had a hard time seeing out the square case could fail if the non-square case works.)