Add support for scad modules

[nickdavies]

This is useful for large models where you want to force rendering regularly and to make the output code more understandable.

I did not attempt to integrate this with the variables logic in the extensions directory though as my usecase was mostly for performance and readability.

[jeff-dh]

I only had a brief look at your code. It looks really nice so far!

Buuuuuuuut unfortunately......

• there is already a similar implementation over here https://github.com/jeff-dh/solidpython2_extensions/blob/master/solid2_extensions/exportResultAsModule.py
• don't know how they compare
• I like the syntax of defining modules as python functions with a decorator and parameters

I like the idea of integrating it into SolidPython2 itself. If you like have a look at the other implementation and adjust if necessary (does your code support parameters?) and I'd be happy to merge a result!

[nickdavies]

Ah I did not see that repo! I will take a look and see if this still makes sense. Currently my code doesn't support parameters, I didn't really need them so I didn't bother but I wouldn't want to regress that if you already have it