ExtrapolationEngine: fix thread-checker warnings

The ExtrapolationEngine was declared ATLAS_NOT_THREAD_SAFE due to the use of the unsafe MaterialEffectsEngine which used several const_cast to update the track parameters in Trk::ExtrapolationCell. This can be avoided by making those parameters non-const to begin with. Obviously, this doesn't make the code thread-safe. But now it is at least const-correct and passes the thread-checker.

Note that the legacy ExtrapolationEngine has only few clients and is superseded by the one from Acts.

cc @christos @ssnyder @pagessin @asalzbur for advice if this is the way we want to go.