Merge remote-tracking branch 'origin/rel/1.26' into staging

Merge remote-tracking branch 'origin/rel/1.24' into staging

Add tests for A-4 and UA-2 to validate the pdfaid:rev field

Update readme

Add Isartor files (#326)

Add test file for 6.4-4

Add test file for 6.2.11.3.3-3

Add tests for rule 8.2.5.20-2

Add tests for rule 8.2.5.20-2 (#323)

PDF/A-2b. Add JPEG2000 test files

PDF/A-4. Update test file with transparency groups

PDF/A-4. Update test files with transparency groups

Update outline of TWG test file (Metadata filter case)

Fix filter tests

Add tests for 8.2.4-2/3 rules