Add support for RO-Crate / ELNFileFormat

[simontaurus]

Export-Workflow:

1. Follow has_part and attachment properties
2. add osw_id = page title to every entity
3. add @context => Category jsonschema slot to every entity
4. extend context
5. apply json-ld expand on every entity
6. add every entity to @graph
7. compact graph with ro-crate context
8. rename "@id" with "url" for Datasets and Files
9. Set @id (=relative path) of all files and sub-datasets (recursive) with ./<parent-name>/name
10. Fetch files and store them in ZIP archive on their relative path
11. add ro-crate-metadata.json to root folder
12. Offer ZIP as download

Mapping (work in progress)
playground