All predicatesShow -- Deal with descriptive nodes in RDF models

To be done
- Define sensible operations on description properties, similar to
- Language selection
Source description_property(?Property)[multifile]
True when Property is used for descritive nodes on a resource. This predicate is multifile.
Source rdf_description(+R, -Description:literal) is nondet
Description is a description for R. This predicate first calls the hook description_hook/2. If this hook fails it produces all property-values for the properties defined by description_property/1 that have a literal value.