WIP: HepMcParticleLink robustness improvements
In HepMcParticleLink_p1 the m_mcEvtIndex corresponds to the position of the GenEvent in the McEventCollection, rather than the event number of the GenEvent. This causes problems in pile-up digitization jobs when many HepMcParticleLink objects cannot form valid links.
This merge request attempts to add a work-around to allow the TP Converter to figure out the correct event number by peeking at the McEventCollecion. This needs careful testing before it is merged. This is merge request follows on from !22633 (merged).