def checksum
      pos = 0
      (numbers+extra_numbers).inject(start_num) do |sum,number|
        pos += 1
        sum + (number * pos)
      end % 103
    end
def checksum
      pos = 0
      (numbers+extra_numbers).inject(start_num) do |sum,number|
        pos += 1
        sum + (number * pos)
      end % 103
    end
Calculate the checksum for the data in this barcode. The data includes
data from extras.