More robust JSON-LD processing

[tijmenbaarda]

Only specific forms of compacted JSON-LD are accepted for now, and this might lead to errors or unexpected results (for example, if rdflib's behaviour changes)

[jgonggrijp]

If we address this, I propose to do it by using the libraries I've been factoring out of READ-IT. Among them, there is a sync method override that can read any form of RDF, not just compact, expanded or flattened JSON-LD but also Turtle, RDF/XML, RDFa and so forth.

[tijmenbaarda]

OK, let's wait for that, then! Because I don't think that we will need to address this issue very soon -- the JSON-LD output from rdflib seems to be quite stable, so some basic validation will do for now...