Call the receiveLocationUpdate() method on each stage when GPS updates

[iswheeler]

Please call the receiveLocationUpdate() method on each stage inside NavigationManager with the current location whenever you receive a GPS update.

For reference, here's the receiveLocationUpdate method (inside DemoStage):

  void receiveLocationUpdate(LatLng newLocation) {
    // Do whatever you need to with the current location.
    // You might want to do some processing (e.g. figure out if the user is close to the end of their walking path) and send a stage event, e.g.:
    //    _controller.add(StageComplete()) // If the user has reached the end!
  }