* At this time, there is no map loaded so no {@link CityModel} object is
* created yet. This happens when a map is imported.
*/
private CityPropertyModels() {
ModelManager modelManager = ModelManager.getInstance();
modelManager.addObserver(this);
}
/**
* Creates a new {@code CityModel} instance, adds it to the
* {@code ModelManager} and connects the number models to this new instance.