Add and specify support for named graphs

[pietercolpaert]

There is currently a proposal to support named graph serializations like TriG and JSON-LD.

The proposal would be to allow something like this:

ex:C1 a ldes:EventStream ;
      tree:member ex:Graph1, ex:Graph2 .

ex:Graph1 prov:generatedAtTime "2022-12-25T12:00" ;
                    foaf:primaryTopic ex:Streetname1 .

ex:Graph1 {
   ex:Streetname1 rdfs:label "Station Street" .
}

ex:Graph2 prov:generatedAtTime "2023-12-25T12:00" ;
                    foaf:primaryTopic ex:Streetname1 .

ex:Graph2 {
   ex:Streetname1 rdfs:label "Station Square" .
}

[pietercolpaert]

As this is an addition to the TREE vocabulary, we might want to discuss this in the TREE hypermedia community group instead.

[pietercolpaert]

Current proposal, to be ratified end of September in the TREE CG, is to always extract the triples in the named graph matching the target of a tree:member. That will allow the initial proposal to work.