Run test_pythiadecays.sh to validate decays in FastSim with particle gun samples. 
This script first defines a list of particle ids.
Then it performs the following for each particle id in the list:

1. A CMSSW cfg file is created that configures CMSSW to generate events with a particle gun,
configured for the respective particle id. The particle gun does not perform any decays.
In other words all decays are generated inside FastSim.

2. Run CMSSW with this configuration

3. Analyse the output of the above
   - create histograms with the generated life times, masses and branching ratios
   - create histograms with the predicted life times, masses and branching ratios,
     with predictions taken from the pythia8 particle data table
These histograms are provided for particle ids specified inside the analyzer
(FastSimulation/ParticleDecay/plugins/TestPythiaDecays.cc)
This analyzers tracks not only the decays of the initial particles, comming from the particle gun.
It also tracks all subsequent decays.

4. Draw comparisons between the generated and the predicted observables.
   Find results in figures/pgun_<PID>/*.png

NOTE:

i)
the mass of the original particle, comming from the particle gun, is defined outside the FastSim decay machinery (in the particle gun itself)

ii)
the life times of all particles are defined outside the decay machinery, in the ParticlePropagator class