HepMcParticleLink fixes for 21.9
Expand HepMcParticleLink unit testing to match master.
Update the HepMcParticleLink class and the HepMcParticleLinkCnv_p1,
HepMcParticleLinkCnv_p2, SiHitCollectionCnv_p3 and TRT_HitCollectionCnv_p4 classes
so that the expanded HepMcParticleLink unit tests pass.
Should help fix ATLASSIM-4712.
Edited by John Derek Chapman