Enable different initial encoding for datamatrix

[flaviencrochard]

Description

Datamtrix can be fully encoded with different encoding. The actual parameters in constructor don't allow that for now.

For instance :
$data = "IS010000000000000000000000S1173082622170111S21.2.250.1.213.1.4.10S3JEAN-MICHEL" . chr(29) . "S4INS-FAMILLE-UN" . chr(29) . "S5MS607-08-1973S726221";

I nned to fully encode that string in C40 (not related to the chr(29) aka GS character)

So here is this pull request to add a third parameter in constructor to set the initial encoding, according to the constant declared in Data class.

Examples :

DATAMATRIX,S / DATAMATRIX,S,N,0 => datamatrix square in ASCII encoding
DATAMATRIX,S,N,1 => datamatrix square in C40 encoding
DATAMATRIX,R,N,5 => datamatrix square in BASE256 encoding

I noticed than on master branch, the test "Test\BarcodeTest::testGetPng" is broken : filename expected isn't correct. So i couldn't check the first item in the checklist
...

Checklist:

• The make buildall command has been run successfully without any error or warning.
• Any new code line is covered by unit tests and the coverage has not dropped.
• Any new code follows the style guidelines of this project.
• The code changes have been self-reviewed.
• Corresponding changes to the documentation have been made.
• The version has been updated in the VERSION file.

Type of change:

• Bug fix (non-breaking change which fixes an issue) → The patch number in the VERSION file has been increased.
• New feature (non-breaking change which adds functionality) → The minor number in the VERSION file has been increased.
• Breaking change (fix or feature that would cause existing functionality to not work as expected) → The major number in the VERSION file has been increased.
• Automation.
• Documentation.
• Example.
• Testing.

[CLAassistant]

All committers have signed the CLA.

[nicolaasuni]

@flaviencrochard
Thank you for your contribution.
I manually merged this PR and refactored it to use strings instead of integers:
#87