More methods for common interface between FitNode and PrFitNode

Another set of methods introduced for LHCb::FitNode and PrFitNode to enable templating of AlignAlgorithm. Furthermore, the function for obtaining a range of LHCb::FitNodes was introduced to LHCb::KalmanFitResult as well.

As the two FitNode types work internally in a different way, consistent and rational unification of some of these methods is challenging. Therefore, any suggestions for improvement are welcome.

Note that in Alignment classically smoothed state (and it's covariance) are needed. With PrKalmanFilter one has to configure the smoother before track fit and so there is no choice between classical and bi-directional smoothing afterwards which is not the case for TrackMasterFitter (and LHCb::FitNode).

@ausachov, @graven, @wouter, @gunther