Python bindings, continued

[shtrophic]

Hello there.

This PR continues the work of #57 which got abandoned. Apparently, the last state before closing was to resolve the installation of the native python module, which is now solved by simply adding an install()-target in pysgm/CMakeLists.txt. Since no python wrapper code is necessary, __init__.py and setup.py is not needed. Just putting the module into its place is enough.

Note that now, pip install . won't work either. For installation, a make install is enough.

[shtrophic]

Building a python wheel now works via python -m build --wheel --no-isolation.