Unify BestPV calculations

The "BestPV" is calculated in:

• https://gitlab.cern.ch/lhcb/Rec/-/blob/master/Phys/FunctorCore/include/Functors/Common.h#L269
• https://gitlab.cern.ch/lhcb/LHCb/-/blob/master/Event/RecEvent/include/Event/PrimaryVertices.h#L245
• https://gitlab.cern.ch/lhcb/Rec/-/blob/master/Phys/DaVinciFilters/src/CopyParticlesWithPVRelations.cpp#L119

these calculations need to be unified (and performed by common code).