moved checksum function to its own interface

its not needed that all barcodes have an integer checksum
so it should not be needed in every implementation.

ean/encoder.go

@@ -158,7 +158,7 @@ func encodeEAN13(code string) *utils.BitList {
 }
 
 // Encode returns a EAN 8 or EAN 13 barcode for the given code
-func Encode(code string) (barcode.Barcode, error) {
+func Encode(code string) (barcode.BarcodeIntCS, error) {
 	var checkSum int
 	if len(code) == 7 || len(code) == 12 {
 		code += string(calcCheckNum(code))
@@ -175,12 +175,12 @@ func Encode(code string) (barcode.Barcode, error) {
 	if len(code) == 8 {
 		result := encodeEAN8(code)
 		if result != nil {
-			return utils.New1DCode("EAN 8", code, result, checkSum), nil
+			return utils.New1DCodeIntCheckSum("EAN 8", code, result, checkSum), nil
 		}
 	} else if len(code) == 13 {
 		result := encodeEAN13(code)
 		if result != nil {
-			return utils.New1DCode("EAN 13", code, result, checkSum), nil
+			return utils.New1DCodeIntCheckSum("EAN 13", code, result, checkSum), nil
 		}
 	}
 	return nil, errors.New("invalid ean code data")