Add initial setup.py

[stuaxo]

Initial setup.py

Don't merge, pull request for conversation

This calls cmake to build nanogui if you run python setup.py install.

It is not useful yet for at least these reasons:

Needs to create a package containing the .so that can be installed.

It also probably makes sense to separate the python part and have that use nanogui as a subproject.

I got stuck on both the above as I'm a bit weak on native libraries for python.