Fix some thread safety issues in TrackingTools/GeomPropagators

This pull request fixes some, but not all, of the thread safety issues in TrackingTools/GeomPropagators that were found by the static analyzer.
Four const member functions that returned a non-const pointer to a data member of the class were modified to return a const pointer instead.
Four data members each had an unnecessary mutable qualifier that was simply removed.
Tested with checkdeps -a and the relval matrix.