Draft: Dataset add named graph

[Murloc6]

This is a draft PR to add to the discussion from #2591

[aucampia]

Thanks for the draft PR @Murloc6 - I'm not sure that the setter for identifier will interface properly with the backing store.

I'm also not entirely sure about named graph, I think we need to ensure the semantics is clear for this in terms of pre and post conditions, but if it will ensure all triples in g will go into the graph named named_graph_identifier then it is good.

[aucampia]

Suggested change

	self, named_graph_identifier: IdentifiedNode, g: Optional[Union[_ContextIdentifierType, _ContextType, str]]
	self, identifier: IdentifiedNode, graph: Optional[Union[_ContextIdentifierType, _ContextType, str]]

I'm not sure why g/graph is not of type Graph though, which it ideally should be. It could also maybe be worth it to take Iterable[_TripleType] as type.

[namedgraph]

Can we add graphs: Dict[IdentifiedNod, Graph] to Dataset instead of using Graph.identifier?

[aucampia]

Can we add graphs: Dict[IdentifiedNod, Graph] to Dataset instead of using Graph.identifier?

We should get rid of Graph.identifier but Dataset.graphs that would have to probably be a property and I would start with making it read only, it would have to interface with the store somehow.

[coveralls]

coverage: 90.636% (-0.01%) from 90.65%
when pulling 761e79d on Murloc6:dataset_add_named_graph
into 14d1006 on RDFLib:main.

[nicholascar]

Maintainers will try to incorporate all these changes into a new Dataset API scheduled for RDFLib 8.x due for release in late 2024