Draft: Adapted secondary origin labeling scheme in InDetTruthOriginTool

This is a small overhaul of how the detector simulation secondaries are categorized in the InDetTruthOriginTool.

Before the condition separating decays in flight (or long lived particle decays) and hadronic interactions (with material) was: For a decay the parent has exactly 2 children and for a hadronic interaction the parent has >2 children. This was a good approximation, but not really ideal (e.g. some strange hadrons decaying with a non-negligible branching ratio in more than 2 particles).