Support SIMD/POD containers and vectorised selections in new functors

The Rec MR has most of the description, go read that.

This MR:

• Gives ::LHCb::Pr::Velo::Tracks an empty() method so it can be treated more like a standard container.
• Gives Vec3<T> some accessors that are capitalised the same way as Gaudi::XYZVector (probably better to do a global rename to one convention, but this was easier.
• Adds more accessors to the SOA::Container skin that wraps a v2::Track.

Goes with Rec!1601 (merged) and Brunel!830 (merged).

cc: @ahennequ @sponce @ibelyaev @apearce @sstahl @graven