Write a function filter_movies that takes as parameters a dictionary of movie-to-average rating (same as for the popularity based function above), and a threshold rating with default value of 3. The function should filter movies based on the threshold rating, and return a dictionary with same structure as the input. For example, if the threshold rating is 3.5, the returned dictionary should have only those movies from the input whose average rating is equal to or greater than 3.5.