getAverageTime(startStation, endStation) {
    const [rounds, totalHours] = this.averageTime[`${startStation}-${endStation}`];
    return totalHours / rounds;
  }
Time O(1) | Space O(1)
Returns the average time taken to travel between two stations.
@param {string} startStation - Start station name
@param {string} endStation - End station name
@return {number} - Average time in hours