Use Case: Building combined knowledge graph of nested RO-Crates

[stain]

If we have #296 and multiple RO-Crates in some ecosystem, it is desirable to query them together. For this it would be good with tooling that can follow every RO-Crate reference and merge into a single knowledge graph.

This would need to merge identifiers as in https://www.researchobject.org/ro-crate/specification/1.2-DRAFT/appendix/relative-uris.html#handling-relative-uri-references-when-using-json-ldrdf-tools and also https://github.com/workflowhub-eu/workflowhub-graph/