Python3 compatibility

Looks like with these changes pyastratoolbox works on both python2 andpPython3. Though, it the first time I ported anything from python2 to python3, keeping compatible with python2, so code review is needed.

Also I noticed that dimension order in examples/s006_3d_data.py is not compatible with my the astra version I use now. I put these changes into this pull request.