Celeritas, master branch (2023.03.01.)
This is a first attempt at building Celeritas as an external project for Athena (and eventually AthSimulation.
I had to adjust the VecGeom and Geant4 builds to make it possible to build Celeritas. And even with those modifications, the way that HepMC3 is picked up by the build is still pretty fragile. But I guess as a first exercise this could be good enough.