SWIG: cleanup

remove commented code
fix SWIG warning 206
SWIG: fix compilation issue with SWIG 2.0/Python 2.7

Use SWIG_NewPointerObj instead of the variant that should only be used
in generated code !
cmake: fix SWIG Python module compilation

By default, cmake takes the "newer" python installation available.
Since it could be python 3.x, we force by default using python 2.x
option ENABLE_PYTHON3 can force the use of python 3.x back

At the moment, we can't compile both version at the same time, needs
some more hacking on the build system.

fix #293