Migrate more v2::Particle containers to use SOACollection

Right now there are several draft/proof-of-concept SOA event model classes that explicitly use std::vector storage: RichPIDs, AssignedMasses, ParticleIDs, CombDLLs, Muon::PIDs, and BestVertexRelations.

Since these were added, a common utility for writing such containers was added: SOACollection. This is already used to define the Composites class and several track classes used in the pattern recognition. Work is ongoing in !2811 (merged) to reduce the amount of boilerplate code that is needed to use SOACollection.

The classes mentioned in the first paragraph should be migrated to use SOACollection. Once that is done, some further cleanup can be done, for example the LHCb::v2::Event::safe_simd_capacity<T>( capacity ) helper function can be removed.