rdf_description.pl -- Deal with descriptive nodes in RDF models

To be done

- Define sensible operations on description properties, similar to rdf_label.pl.

- Language selection

 description_property(?Property)[multifile]

True when Property is used for descritive nodes on a resource. This predicate is multifile.

 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.