HepMC3 migration and multi-threaded generation phase

Migration of the generation phase to HepMC3. HepMC events are currently stored temporarily on the TES as std::vector<HepMC::GenEvent>

Done:

• Migration of Gen/Generators, Gen/LbPythia8 and Gen/LbPGuns
• Can generate events and fill the monitoring diagrams
• Modernization of involved algorithms to be able to read non-DataObjects from TES
• Thread-safe GenCounters using atomics
• Algorithm to optionally write HepMC into ROOT (either TTree or native format), ASCII or HEPEVT files.
• Thread safe random numbers using engines local to algorithm's executes
• Multi-threaded generation phase (with locked access to Pythia)
• Validation executable to compare generated files to check reproducibility

ToDo:

• Modernize all involved tools (use of Gaudi::Property etc)
• Remove remaining dependence on LHCb::HepMCEvent (mainly due to the particle Status codes)

Once ready, I will rebase this branch to repackage and squash commits to a more logical structure