restore integration with `morph-kgc`

[ceteri]

Restore the really excellent kglab.KnowledgeGraph.materialize() capability after morph-kgc resolves its unit test conflicts with all the breaking changes due to rdflib v7.x and subsequent dependency pinning.

This integration with morph-kgc will be missed, and we hope to restore this capability soon!

See:

• RDFLib 7.3.0 breaks some unit tests morph-kgc/morph-kgc#345
• https://github.com/DerwenAI/kglab/blob/main/morph_kgc.md

cc: @arenas-guerrero-julian