make it possible to build mussa with python

make it possible to build mussa with python
(well actually this patch will now default to building with python)

Getting this to work required popping the qui/mussagl target down to the
root of the CMakeLists.txt because the executable depended on both the
mussa_qui library and the mussa_py library and the mussa_py code depended
on the mussa_qui library. Which meant that the simple linear ADD_DIRECTORY
structure wouldn't work. ADD_DIRECTORY(py) had to be between building
the qui library and the executable.

as a bonus this actually puts the executable in a more comprehensible
place.