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.
My motivation is that I use the zbf-loading and saving features (and the unit converting feature) to do automated analysis of many beamfiles of different formats. My computer at home and the cluster I once used runs on linux. Of course trying to support multiple OSes is lots of extrawork. I'm not sure if I can help you testing and fixing on unix in the future so it's up to you if you want to keep the os-independence-features.

`_global_ddeclient_load` is set `True` if the ddeclient could be imported. I thought this could be useful for later checks. Actually `_global_ddeclient_load = True` must be set at the last line of the try block. I'm sorry my mistake. I will fix it till next week.

According to what's left from question2:
>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" ? 
I'd say yes, one could handle it like this :-)