#223: More unification using ADL
Next round of changes for #223:
- use Vec3 for position and three-momentum, adapt accessors
- use unqualified dot for dot product
- more use of threeMomentum
- add slopes for those cases where the momentum magnitude is unknown or not required
- reduce duplication of code in PIDx functors
- remove no longer needed duck-typing accessors
- add referencePoint, endVertexPos and threeMomentum support to TrackCompactVertex
must be applied in conjunction with LHCb!3359 (merged) and Phys!1028 (merged)