def round(digits = nil)
      rounded_value = digits ? value.round(digits) : value.round
      self.class.new(rounded_value, unit)
    end
def round(digits = nil)
      rounded_value = digits ? value.round(digits) : value.round
      self.class.new(rounded_value, unit)
    end
Round the measurement value. Delegates to the value's class.
 @return [Integer, Float]
 @api public