WIP: MultiTrajectory improvements

This MR does the following

• Allow addTrackState(const TrackState&), so from an existing "regular" TrackState object, to also accept a mask to enable allocation of additional components, over what the TrackState contains.
• Right now, the source data will only be copied, if the mask has caused allocation for that component. @xai: do you think it makes more sense if the components present in the source TrackState always get allocated? I could generate a mask automatically, and OR it with whatever is explicitly given on top
• Use a strong type instead of a bitmask, so that the mask is not autoconvertible from integer types
• Add a method that allows copying between track states. I still need to think about/discuss this