A hack for PDF/A-3 compliance

[akarrenbauer]

• adds /AF key followed by an array of embedded objects to the Catalog
• adds /AFRelationship key followed by the appropriate arguement, which shall be provided by the user via the new option "afrelationship", which shall be one of /Data, /Source, /Alternative, /Supplement, or /Unspecified.

This patch relates to Issue 37. With this patch, the mentioned examples can be made to pass validation for PDF/A-3b compliance (tested with veraPDF) by adding the new "afrelationship" option.

[davidcarlisle]

Thanks for this, we are about to spit the package into a separate repository so merging now, before re-arranging the git sources.