Support JSON-LD catalogs with @graph

[seitenbau-govdata]

Adds support for parsing JSON-LD catalogs containing @graph elements.

Links to open issues in rdflib-jsonld relating this problem:

• Triples not loaded when using @context and @graph RDFLib/rdflib-jsonld#40
• not loading jsonld file RDFLib/rdflib-jsonld#53

The suggested "solution" is to use rdflib.ConjunctiveGraph() instead of rdflib.Graph().

[metaodi]

@seitenbau-govdata is there any downside in using ConjunctiveGraph? So this should work for all our cases? I see that all tests pass, so I guess everything is fine.

[seitenbau-govdata]

@metaodi We have had the same thoughts. Ultimately we don't know it if something special is not working with the ConjunctiveGraph. So, we have to trust the implemented tests that all relevant use cases are covered.

[seitenbau-govdata]

@metaodi What do you think about the pull request? Maybe it's helpful to have another opinion.
@amercader Please let us know your view about the pull request.

[metaodi]

I like it and think it's a good solution. I'd say let's risk it and test in all our installations, if something is broken, we can still revert if needed.

[amercader]

Tests with the alternative JSON-LD syntax seem to pass, so let's merge it

[seitenbau-govdata]

Thanks @metaodi and @amercader !