While commissioning, it was noticed the eta calculation for VELO tracks, based on the momentum, gave nan. With this MR, the state() method on VELO tracks returns a KalmanVeloState instead of a KalmanState (using the conversion already available) and a method for eta calculation based on the slopes is added. Nothing changes for eta calculation of Basic/CompositeParticle.
By monitoring VELO tracks eta on simulation, the obtained distribution is shown below 
Also, Velo::Track and Velo::TrackFitParameters structs, unused, are cleaned