Draft: Enable G4HepEm in Athena

G4HepEm is an optimised library for electromagnetic physics in Geant4: https://github.com/mnovak42/g4hepem

This MR creates an experimental physics list with G4HepEm enabled, to allow for testing and debugging. Simulation jobs run successfully, albeit with significant physics changes w.r.t. standard fullsim.

An open question is how to include the G4HepEm library in Athena. At the moment, a conversion script is provided to check out the git repository and tweak the code to compile it as an Athena package. In the future, G4HepEm might be integrated with Geant4 or provided through AtlasExternals.