TrackSlimmingTool: allows slim/skim non const Trk::Track directly

The interface allows for the following

• slimTrack we can do whatever we want to a non-const track.( Still a bit less efficient than it should but this is the best I could come up with current Trk::Track design).
• slimCopy . For a const track is fine to make a copy and then slim the copy and return it
• slim This is not MT safe, the reason being that setting hints in reality changes information that ,for now at least, is actually attached to the const TrackStateOnSurfaces which are attached to a const Track.

Mentioning @ssnyder

I also try to use this in the EMBremCollectionBuilder , i.e slim the non const Tracks we create there, so mentioning also @amorley