Use Case: Convert RO-Crate to/from DCAT-AP

[stain]

As an EOSC user I am encouraged by funders to use DCAT-AP 3.0 for publishing datasets. I want to convert the RO-Crate metadata for a published crate to a DCAT description.

1. https://schema.org/Dataset maps directly to DCAT Dataset but in a different namespace. Mapping https://ec-jrc.github.io/dcat-ap-to-schema-org/ shows how to map, but we need programmatic version
2. A RO-Crate for DCAT must be published in a way that can fit in a DCAT DataCatalogue and have a distribution.
3. DCAT expects typically format Turtle (although this is never mandated?) not JSON-LD. In addition the URLs must be absolute, so the RO-Crate must be converted to Detached.
4. The inner details of an RO-Crate e.g. author per file should not be included in the dcat, only the ./ level root descriptions need to be reflected in DCAT.
5. Some additional DCAT-AP requirements are not required in Schema.org but can be expressed by given mapping. A corresponding "DCAT-compatible RO-Crate" profile may be needed.
6. Note that DCAT-AP is a particular profile of DCAT, there are also more domain-specific profiles like https://healthdcat-ap.github.io/ and https://semiceu.github.io/GeoDCAT-AP/

Relates to #220