Const function must return const ptr for thread safety

clone() is a const member function of TrackFitter that returns a non-const pointer through which a non-const data member of the invoking object can be accessed.  This is because the function, under certain conditions, does a shallow copy (It is poorly named).  The pointer is never used to modify any member.
This pull request (for thread safety) changes the function to return a pointer to const, and propagates the const qualifier through another package so that everything compiles.