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.
I didn't anticipate that anyone would use PyZDDE in non-windows systems. But I think I understand your motivation, and I think it is good idea. I have the following two thoughts:

1. I don't quite understand the purpose of the variable ``_global_ddeclient_load``.
2. Do you think that a message like "DDE client couldn't be loaded. All functions that start with "z" or "ipz" will not work" ?