Replace dcterms:hasPart with a specific property

[andrea-perego]

dcterms:hasPart is re-used in DCAT as the generic relationship between a dcat:Catalog and the listed dcat:Resource's, i.e., to specify that a given element (dcat:Resource) is in a given set (dcat:Catalog).

The problem is that dcterms:hasPart, as per its DCTERMS definition, may indicate either an element in a set or a subset. This may raise issues in the use of this property, e.g., to specify relationships between a catalogue and its subsets (sub-catalogues), as reported in #1454

The proposal is as follows:

1. Revise §6.3.3 by replacing dcterms:hasPart with a specific property dcat:resource. It will be sub-property of dcterms:hasPart, with domain dcat:Catalog and range dcat:Resource.
2. Revise properties dcat:dataset, dcat:service, and dcat:catalog to make them sub-properties of dcat:resource.
3. Add dcterms:hasPart under dcat:Resource.

The proposal is implemented via PR #1470

[riccardoAlbertoni]

I am in favour of this proposal, doing so we leave dcterms:hasPart for Loosely structured catalog, and we have specific subproperties for more specific use cases.