removeZeros() {
    if (!this.isValid) return this;
    const vals = removeZeroes(this.values);
    return clone(this, { values: vals }, true);
  }
Removes all units with values equal to 0 from this Duration.
@example Duration.fromObject({ years: 2, days: 0, hours: 0, minutes: 0 }).removeZeros().toObject() //=> { years: 2 }
@return {Duration}