List children based on all targetclasses in schema

[dennisvang]

Basically this is a refactored version of the solution suggested by @qplevier.

Whereas the current implementation picks the first item from the list of targetClassUris, we now check all specified target classes, i.e. both dcat:Resource and its subclasses. As a result, the list of matched subjects is likely to contain duplicates, e.g. one for dcat:Resource and one for dcat:Catalog, but these are removed with the help of Set.

Also added some type specifications, for clarity, and changed the arrow function argument.

fixes #210

Rationale

As far as I can see, the goal in EntityConfig.createChildrenList() is to create a list summarizing all the child resources.
For this reason it needs to obtain all subjects matching the type dcat:Resource or one of its subclasses.

Because the data graph only contains triples describing the child resources, it should be sufficient to grab unique subjects matching any of the target classes defined in the active resource definition.

Notes:

• The FDP reference implementation specifies both <subject> a dcat:Resource, dcat:Catalog. This seems redundant, knowning that dcat:Catalog is a subclass of dcat:Resource. However, the client is not aware of the DCAT class hierarchy, because there are no explicit rdfs:subClassOf statements in the data graph, and it has not loaded the dcat spec.

[dennisvang]

Thanks for the review @kburger. 🙂
I'll merge this and make a new release.