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.
Edited by John Derek Chapman