message: Use reference syntax for tile
  This makes the two addStructure calls a little more consistent.
diff: @@ -192,8 +192,8 @@ void MapViewState::onDiggerTaskComplete(Robot* robot)
  throw std::runtime_error("MapViewState::onDiggerTaskComplete() called with a Robot not in the Robot List!");
  }
  - Tile* tile = mRobotList[robot];
  - const auto position = tile->xyz();
  + Tile& tile = *mRobotList[robot];
  + const auto position = tile.xyz();
  if (position.z > mTileMap->maxDepth())
  {
  @@ -210,7 +210,7 @@ void MapViewState::onDiggerTaskComplete(Robot* robot)
  AirShaft* as1 = new AirShaft();
  if (position.z > 0) { as1->ug(); }
  - NAS2D::Utility<StructureManager>::get().addStructure(as1, tile);
  + NAS2D::Utility<StructureManager>::get().addStructure(as1, &tile);
  AirShaft* as2 = new AirShaft();
  as2->ug();
  