Gant4 and G4PY installation via SPACK

PR tries to address the installation of g4py via spack (which can already be used to install geant4). This could be an alternative for MacOS users.

However there are some challenges due to specific dependencies that are required by g4py and not geant4.

Specifically, we are unsure which optional dependencies of geant4 are transitively required to build the python bindings. Also, Python3 support to link Boost.Python seams to be problematic and help is appreciated.

Please feel free to take a look at that PR and let us (me and/or know how we can fix them.

Thanks, Diana