mapUnits(fn) {
    if (!this.isValid) return this;
    const result = {};
    for (const k of Object.keys(this.values)) {
      result[k] = asNumber(fn(this.values[k], k));
    }
    return clone(this, { values: result }, true);
  }
Scale this Duration by the specified amount. Return a newly-constructed Duration.
@param {function} fn - The function to apply to each unit. Arity is 1 or 2: the value of the unit and, optionally, the unit name. Must return a number.
@example Duration.fromObject({ hours: 1, minutes: 30 }).mapUnits(x => x * 2) //=> { hours: 2, minutes: 60 }
@example Duration.fromObject({ hours: 1, minutes: 30 }).mapUnits((x, u) => u === "hours" ? x * 2 : x) //=> { hours: 2, minutes: 30 }
@return {Duration}