code: void retrieveRouteFor(OfflineRoute offlineRoute, OnOfflineRouteFoundCallback callback) {
      new OfflineRouteRetrievalTask(navigator, callback).execute(offlineRoute);
    }
docs: Uses libvalhalla and local tile data to generate mapbox-directions-api-like json
  
  @param offlineRoute an offline navigation route
  @param callback     which receives a RouterResult object with the json and a success/fail bool