Use unique_ptr for tracks in MuonTrackSteeringTools

This MR switches to using unique_ptr for Track and MuPatTrack objects. In particular, MuPatTrack now stores a unique_ptr to the Track, rather than an (unnecessarily) const bare pointer. This should simplify the memory management for tracks and eliminates the need for some const_casts. Also, the MuonTrackSelectorTool previously used the ITrackSelectorTool interface, which did not include a non-const method. Since only one of the interface methods was actually used, and the MuonTrackSelectorTool itself is only used in MuonTrackSteering, I removed the interface and moved the code for the tool to MuonTrackSteeringTools.