Add RdfReader supporting JSON-LD, TriG and RDF/XML

[niklasl]

This is mainly in preparation for ingesting RDF/XML (part of general importer improvements for systems librarians).

Feature includes:

• Add RdfXmlToJsonLdParser (and EmbedBlanks, currently only used for that)
• Adapt TrigToJsonLdParser (performs expand-and-compact; could be extended to flatten and re-embed blanks)
• Adapt create-from-rdf.groovy example script
• Unit tests
• Example source data

Example test creation (replace .rdf with .ttl or .jsonld for the different example input formats):

$ cd whelktool/
$ java  -Dxl.secret.properties=../some-secret.properties \
        -Drdfdata=scripts/examples/records/libris-example-record.rdf \
        -jar build/libs/whelktool.jar --dry-run --step scripts/examples/create-from-rdf.groovy

(Each file contains one main entity (and some extra meta for the record). It will fail since create requires loud mode, but choose p to print the resulting jsonld form that would have been created in XL.)

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Following this, we want:

1. Some discussion about useful IRI handling.
2. Add the script logic as a buitlin whelktool operation? (Another such candidate is apply-rdf-patch.groovy.)
3. One or two new ways to ingest record sets (including simply loading them as sets of files); reasonably added to DatasetImporter. We can do this as a follow-up PR.

[olovy]

LGTM!