Merged requested to merge kalman_master into master
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).