Review JSON-LD support and pyLD

[rob-metalinkage]

At RDFLib planning meeting on 28/04/2020 a number of issues about JSON-LD support were canvassed. Before making specific issues about functionality it was determined that the pyLD https://github.com/digitalbazaar/pyld team should be consulted, as it was desirable to have a single canonical codebase for this under the RDFlib umbrella.

Specific functional requirements to consider include:

1. Can we force use of specific context URIs during serialisation, driven by imports and content present

2. can we deserialise plain JSON using an external context ? Can we inhance using Json to JSONLD to RDF Converters? #928