unit_mex_overdrive.lua: add UnitRulesParam ("gridNumber") for future AI use, and some cleanup

I remember that there's optimization that make mex a pylon (I forgot which ancient commit). While adding the UnitRulesParam() I saw some code that still think mex as non-pylon unit (loops that search which pylon had the mex), so I remove that code and replace with straightforward code that assume mex == pylon (less code). 

Some simple test look okay (based on grid colour & tooltip)