MCTruthClassifier: fix potential infinite loop
- Fix potential infinite loop if a particle doesn't have a parent.
- Refactor to properly check cases with multiple parents (e.g. in 2->N splittings)
This is a more general solution to the issues outlined in !52281 (closed). (cc @ssnyder )
New logic validated against old one using PP8 ttbar and Sherpa W->taunu.
Edited by Christian Gutschow