Merged requested to merge averbyts/athena:removetph into master
Simplify xAOD true particle
Tag @abuckley @christos @jchapman
The idea is to create a member function instead of multiple calls to
MC::isGenStable. Therefore, the code that defines
isGenStable is locatized in that function. This also removes the need of the header
TruthParticleHelpers.h, which is not used anywhere else.