BALL.py should be added to PYTHONPATH automatically

[cschaerfe]

1. Cmake does not use contrib sip, but that one that was installed with python (e.g. when using anaconda) -- does it find the contrib sip?

2. BALL.py is built, but BALLView cannot find BALL module correctly, leading to the following message in BALLView python console:

> No Python support available:
import BALL

These two things are likely connected and thus described here together.

[chahuistle]

Still not sure what's the problem with issue (1), but issue (2) has to do with environment variables (PYTHONPATH) not being properly set.

[tkemmer]

~~(1) Does not seem to be a problem anymore.~~

We will consider (2) as a build system feature for BALL 2.0.

[tkemmer]

Reopening (1) as separate issue.