Add possibility to build pythia8 with hepmc2 or with hepmc3, add hepmc=2 to...
Add possibility to build pythia8 with hepmc2 or with hepmc3, add hepmc=2 to previous toolchains for compatibility. This will help to mitigate problem with hashes
For all new HepMC3 layers we need to build Pythia8 with HepMC3, as before we built it with HepMC2 we added hepmc=2 for all previous layers to preserve original behaviour