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.
Few comments here:

1. I think this kind of function could be useful. However, I think converting between mm, cm and m is so trivial that it don't call for a function. Also, meter to meter is completely redundant. 
2. I think a function to convert between just mm to/from inch might be more useful... something like ``milli2inch()`` and ``inch2milli()`` (similar to the functions ``deg2rad()`` and ``rad2deg()`` of Numpy.
3. The name of such functions must not start with "z" as such function will typically require Zemax to be running.