Make TrajectorySmoothers const thread-safe

Most of the TrajectorySmoothers were changing the direction of their
Propagators in a const function. That is not thread safe. To fix the
problem the TrajectorySmoothers now hold two Propagators which only
differ based on their propagation direction.
Merge pull request #3721 from Dr15Jones/constThreadSafeTrajectorySmoothers

Multithreading -- Make TrajectorySmoothers const thread-safe