Several fixes for the PAPA Framework (squashed)

- Add protection in CheckParticleSource for Tracks without MC Track
- Add label for EventStartTime variable
- Fix mass dependent weighting for single tracks from thermal source
- Fix typo in mother pdg for qgp radiation
- Fix weighting of pluto particles from different mothers
- Fix mass dependent scaling for comb. pairs from thermal sources
- Assign mctrack pointer directly instead of creating it as nullptr first
- Move include TClonesArray outside of ifndefin PairAnalysisMC, remove from PairAnalysisPair
- Dont define MCtrack as nullptr but assign it directly
- Change uses of 0x0 to nullptr

See !1261 for original description and discussion