mapEndpoints(mapFn) {
    return Interval.fromDateTimes(mapFn(this.s), mapFn(this.e));
  }
Run mapFn on the interval start and end, returning a new Interval from the resulting DateTimes
@param {function} mapFn
@return {Interval}
@example Interval.fromDateTimes(dt1, dt2).mapEndpoints(endpoint => endpoint.toUTC())
@example Interval.fromDateTimes(dt1, dt2).mapEndpoints(endpoint => endpoint.plus({ hours: 2 }))