Modify EvtPythiaEngine to properly initialize EvtHmePythia model. Clang-formatted testDecayModel. Added HmePythiaTau1.json for tau -> e anti-nu_e nu_tau testing.

The initWaves function in pythia8/include/Pythia8/HelicityMatrixElements.h
needs to be made public (from protected) before we can use it.