Add posCovMatrix() accessor to v2::RecVertex, tweaks to SIMDWrapper et al. and expand PrIterableTracks

• The new accessor is needed for the new functors' implementation of BPVFDCHI2, which is needed for the TwoTrackMVA MVA.
• Minor tweaks to SIMDWrapper, Vec3 and LHCb::Pr::Fitted::Forward::Tracks to allow them to be used by generic code
• Expand the helpers in PrIterableTracks.h to enable iteration over LHCb::Pr::Fitted::Forward::Tracks

Goes with Rec!1659 (merged) and Moore!223 (merged).