FastSim: ParticleDecays: fixed bugs to pythia8 option, made pythia8 option default
fastsim : decayer : fix bug leading to double decays
fix to pythia8 option for FastSim decays, first commit of validation code
extended and fixed validation code
Merge pull request #3989 from lveldere/fastsim-decayer-fixes

FastSim -- Switch to Pythia 8 for FastSim decays