Bugfix of problem reported by Mathias Fröhlich: Ground network trace()

Bugfix of problem reported by Mathias Fröhlich: Ground network trace()
algorithm caused a program crash. Because there is always one waypoint more
than there are routes, the trace function should only pop_back the final route
entry at search depths of one or higher. I also added a lot of of additional
safeguarding code, due to the fact that the new trace algorithm was
apparently not as stable as I'd hoped it would be. ...