DCAT validation results

Now that the OGC API DCAT feed can export an entire catalog (see also https://github.com/geonetwork/geonetwork-microservices/pull/59), it would be great to pick up the work left on the DCAT converter.

The [opendata.vlaanderen.be/validator](https://opendata.vlaanderen.be/validator/) DCAT Validator gave me the following suggestions:
* **URLs as resources**: now URLs are literals `https://example.com/download` but should be resources ``

* **email addresses with mailto://**: also email addresses should become resources. `noreply@example.com` should be resources with mailto-protocol indicated ``

* **language tags**: dct:title and dct:description literals need a language tag. Ideally this also works with multilingual records.

* **dcat:Catalog**: include a catalog description in the DCAT output. Catalog properties such as dct:license should ideally be made configurable.

* **dct:publisher**: dcat:contactPoint is already provided with various roles, but DCAT also requires a dct:publisher property.

* **dct:accessRights**: the DCAT converter does not seem to map it.

Additionally, also this deserves some further thought:

* **URIs for blank nodes**: Now resources like dcat:Dataset, dcat:CatalogRecord, skos:Concept do not have a URI. For dcat:Dataset and dcat:CatalogRecord the URI could perhaps be made configurable. For skos:Concept, the URI could be taken from the link (gmx:Anchor) if available.