Add checksum calculation to relevant barcode types

datamatrix/datamatrixcode.go

@@ -40,6 +40,10 @@ func (c *datamatrixCode) At(x, y int) color.Color {
 	return color.White
 }
 
+func (c *datamatrixCode) CheckSum() int {
+	return 0
+}
+
 func (c *datamatrixCode) get(x, y int) bool {
 	return c.GetBit(x*c.Rows + y)
 }