Fixed const correctness for CosmicMuonSmoother

The static analyzer identified that CosmicMuonSmoother was returning
a non const pointer to internal member data from a const function. This
is a thread-safety issue since a routine that has a 'const' CosmicMuonSmoother
can still change the memory owned by the smoother. The fix was trivial since
no other code cared that those pointers were non-const.
The two TrajectoryBuilder classes were passing on one of the non-const
pointers so they also had to be changed to return a const pointer.