add counters to packing of MCParticle and MCVertex
In order to improve the test of MC packing, add some counters to the packing of MCParticle and MCVertex so that it can be tracked how many particles/vertices we pack per event.
Note: having this in place will help decide whether MR such as Rec!3919 (merged) keep the MC packing unchanged. So this should be merged prior (and independent of) Rec!3919 (merged)
Edited by Gerhard Raven