Use Case: Deep archive of RO-Crate with downloaded references

[stain]

trafficstars

As a archivist I want to download every URL-referenced file in hasPart to get a "fat" RO-Crate

Follow https://www.researchobject.org/ro-crate/specification/1.2-DRAFT/appendix/relative-uris.html#converting-from-detached-to-attached-ro-crate on converting from Detached to Attached RO-Crate -- but this should work for all RO-Crates with references.

May need #337 to specify desired filename after downloading.

Out of scope: Nested RO-Crates