Some python3 adaptions, introduced zUnitToMeter, OS independence

This corrects some spelling mistakes, makes the not-zemax-functions usable on unix, restores python3 compatibility and introduces a new Helperfunction to convert units.
Thanks a lot for correcting this, and the rest of ``raise`` messages. It's my bad. Before checking in the code, I didn't test in Python 3. (I usually work in Python 2.7 as I need to use Mayavi). By the way, are you using Python 3?