- Add getModel(ResourceSet resourceSet, Class<T> clazz, EFactory factory,...

- Add getModel(ResourceSet resourceSet, Class<T> clazz, EFactory factory, String fileExtension, Class<R> referenceClazz), a method that Obtains a model of a given type from a {@code resourceSet}, and creates the model if it does not exist yet (and adds it to the given {@code resourceSet}) and that returns {@code null} if the model could not be created.