code: public int getAverageHeadwayForStop (String stop_id, LocalDate date, LocalTime from, LocalTime to) {
          List<Trip> tripsForStop = getTripsForDate(stop_id, date);
  
          return getStopHeadwayForTrips(stop_id, tripsForStop, from, to);
      }
docs: Get the average headway, in seconds, for all trips at a stop over a time window.
  @param stop_id
  @param date
  @param from
  @param to
  @return avg. headway (in seconds)