code: def get_album_time(self, round_number=2):
          
          if not self._track_list:
              self.get_tracks()
          if self._album_time is None:
              album_time = 0.0
              for track in self._track_list:
                  album_time += track.get_track_time_minutes()
              self._album_time = round(album_time, round_number)
          return self._album_time
docs: Retrieves all of the track's length and returns the sum of all
          :param round_number: Int. Number of decimals to round the sum
          :return: Int. Sum of all the track's length