Switch to HepMcParticleLink_p3-based persistency
unsigned int rather than
unsigned short to hold the eventIndex. This is a step towards preventing the aliasing of higher event numbers seen with
HepMcParticleLink_p2. This merge request does not change the transient
HepMcParticleLink class, so it won't fix the problem on its own. The downside is that it will increase the size of events on disk. The size of this effect will need to be determined before the change is merged.
NB This will (temporarily) break the ability to read files produced in 21.9 with other branches.