Data cleaning for license identifiers

[stain]

Many Workflow Hub RO-Crates have used license as a literal string rather than as a @id reference, which mean we get many variants for the same license:

    schema1:license "MIT" ;
# ...
    schema1:license "https://spdx.org/licenses/MIT" ;

https://www.researchobject.org/ro-crate/specification/1.1/root-data-entity.html#direct-properties-of-the-root-data-entity says it should "SHOULD link to a Contextual Entity in the RO-Crate Metadata File with a name and description" meaning it should look like this:

"license": {
  "@id": "http://spdx.org/licenses/MIT"
}

and after parsing to RDF graph:

schema1:license <http://spdx.org/licenses/MIT> ;

https://about.workflowhub.eu/Workflow-RO-Crate/ro-crate-metadata.json also defines the SPDX identifiers with http URIs not https -- this is to be compatible with identifiers used in https://github.com/spdx/license-list-data/blob/main/rdfturtle/MIT.ttl etc.

From this I'm getting the feeling we need two outputs, one "raw" RDF which may be exactly as in the RO-Crate, and one data cleaned. This may just be the named graphs #29 are "as is" and the separate output without named graphs has done such data cleaning -- or added this as a secondary named graph.