Implement support for provenance data

[proycon]

FoLiA v2.0 documents may now carry provenance data, an annotationtype uses either this or the old system of annotators

• [ ] - Implement parsing of provenance data (holding <processor> elements)
• [ ] - Implement parsing of <annotator> elements in the annotation declarations, this is what links declarations to processors in the provenance data
• [ ] - Implement parsing of processor attribute (a common FoLiA attribute that links any element to its processor, analogous to annotator/annotatortype pre-2.0)
• [ ] - Implement serialisation of provenance data
• [ ] - Implement serialisation of <annotator>
• [ ] - Implement serialisation of the processor attribute
• [ ] - Implement an API for setting the processor (see proycon/foliapy#9)

See:

• original issue: proycon/folia#46
• documentation: https://folia.readthedocs.io/en/latest/metadata.html#provenance-data

[kosloot]

In libfolia now