add PV pointer to particle, update v2 PV event model
This combines a few things needed for run-3 PV persistency:
- update and cleanup PrimaryVertexContainer class, streamlined PV unbiasing code
- add add/unpack functions for PrimaryVertexContainer. These are not used at the moment. We will continue persisting v1 PVs but with more proper converters.
- add PV pointer to LHCb::Particle, to be filled by particle maker/combiner. Including relevant updates to persistency.
Closing #257 (closed) Moore#608 (closed)
Partially addressing Rec#363 Moore#139 Moore#604 Moore#613
Needed for Rec!3665 (merged), Moore!2658 (merged) and DaVinci!1013 (merged)