freeTrajectoryState() should return const pointer

freeTrajectoryState() is a const member function of TrajectoryStateOnSurface that returns a non-const pointer to a data member of the class.  The pointer is never used to modify the member.
This pull request (for thread safety) changes the function to return a pointer to const, and propagates the const qualifier through other packages so that everything compiles.
Tested with checkdeps -a and the relval matrix.