`SurfaceReached` aborter is messing with the step size

The `SurfaceReached` is doing basically a navigation procedure in parallel to the `Navigator` and the one which sets the smallest step size wins. This seems error prune to me and like a mixture of concepts. IMO only the navigator should set the step size and the aborter can check if we are on the surface or not.

This could be realized by adding the target surface as an external surface to the navigator state.