Support specification of descriptors for object property collection elements

[ledsoft]

Currently, it is possible to specify a descriptor for an object property collection, i.e., a single descriptor is used for all elements of the collection. However, such functionality is not sufficient.

For example, consider the following class:

@OWLClass("skos:Concept")
public class Term {
    @OWLObjectProperty(iri="skos:broader")
    private Set<Term> parents;

    // ... other attributes
}

If the parent terms belong to different vocabularies, which are stored in separate contexts (RDF named graphs), it is currently not possible to reference them.

It should be possible to specify descriptors for individual elements of the collection