This merge request should go together with Rec!497 (merged) and Brunel!143 (merged).
Essentially, in order for TrackVectorFitter to work while preserving TrackMasterFitter compatibility, some structural refactoring was needed.
- Refactor LHCb::Node to include some LHCb::FitNode definitions.
- Added LHCb::ChiSquare to Event/ (previously defined in Rec::FitEvent/).
- Added chi2 types (TT, downstream, etc.) to TrackFitResult (from FitEvent/KalmanFitResult).