IPropagator returns new TrkParameters , typically owned by the caller. So there is not need for them to be const

The IPropagator interface and implementation always return new parameters.

They are owned by the caller both in lifetime and on if they are to be modifed or be treted as immutable.

Ideally these would return unique_ptr<Trk::Parameters>

For now just remove the un-nneded const to allow also to remove some un-needed const-cast or clones and then delete from clients

Mentioning @amorley as some of these cases were mentioned by him