def unit_str
      rv = @numerator_units.sort.join("*")
      if @denominator_units.any?
        rv << "/"
        rv << @denominator_units.sort.join("*")
      end
      rv
    end
def unit_str
      rv = @numerator_units.sort.join("*")
      if @denominator_units.any?
        rv << "/"
        rv << @denominator_units.sort.join("*")
      end
      rv
    end
Returns a human readable representation of the units in this number.
 For complex units this takes the form of:
 numerator_unit1 * numerator_unit2 / denominator_unit1 * denominator_unit2
 @return [String] a string that represents the units in this number