added doxygen support for python bindings

I introduced the doxygen support for python bindings. I used some of the scripts developped in BornAgain but I had to made a bit of changes in order to fit with nsxlib. I may be wrong but it seems that BornAgain is automatically shipped with python bindings. As it is not our case (so far), I had to change a bit the strategy. Indeed, I had to use a configure_file call in order to modify at configuration time the contents of the pynsx.in file according to the explanation of one doc2swig available here. I also had to modify accordingly some of the swig settings in the corresponding cmake file. The python doc is now part of the pynsx lib and contains the corresponding c++ docstrings (tested on pynsx.Intensity). Please feel free to add some changes if you you think it can be improved.