Outstanding work for Alignment annex

[davebrowning]

Further work after PR #822 which actioned the resolution from 2019-03-06 (now merged).

@dr-shorthair said:

The annexe needs a little more work, to verify that all properties added or modified since the original version are accounted for.
Also - it refers to an RDF representation which axiomatizes the mapping using various SKOS, RDFS and OWL predicates, which might overreach.

[dr-shorthair]

Here's a few proposals:

1. sdo:DataFeed is a dcat:Dataset with dct:accrualPeriodicity <http://purl.org/cld/freq/continuous>

2. dcat:Relationship -- sdo:Role - note however that sdo uses the same existing property name for both the the incoming and outgoing link - see examples in https://schema.org/Role

3. Trying to figure out if there is a mapping between dcat:DataService and sdo:EntryPoint ...

4. prov:wasGeneratedBy -- inverse(sdo:result) which is a bit awkward

5. dct:accrualPeriodicity -- sdo:repeatFrequency

[dr-shorthair]

Some minor updates to the mapping in #865
See https://rawgit.com/w3c/dxwg/dcat-issue849/dcat/index.html#dcat-sdo