Set proper build switches for HepMC and Pythia support
We would like to have HepMC and Pythia support optional: OFF by default, but ON on user's request.
At the moment:
- HepMC is ON by default, we should move it to OFF.
- Pythia is OFF by default, but it silently builds Pythia if Pythia is found
We should have clear switches to have them ON/OFF, without silent builds/detection.
We should also have the HepMC/Pythia features in FSL greyed out when no support for them is built. In that way, users that haven't enabled their build still can see that support for them is possible/available, if enabled.
Note:
All this does not apply to distribution kits: at the GeoModel meetings it was agreed that the idea there is to have the full functionality. So, distribution kits should install all functionalities with all needed dependencies.
TODO: remove the optional flag to optionally disable HepMC3 installation in FSL and FullSimLight Homebrew recipes.