Improve Python Packaging

The current python packaging approach installs gmsh into the wrong directory when using the --user flag, which is default in the current pip. Fixing this was non-trivial, and the changes made the setup better suited for wheel distribution.

The CI now builds both the source and the wheel for 64-bit Linux, and installing from the wheel is much faster than the current sdist install as the wheel does not need to be built when installing, and the PyPi servers seem to have better bandwidth.