Supprot to extraction involving BNodes in TURTLE_ITER and NT

• Extraction works properly even if there are BNodes as target instances or in the neighborhood of those whenever a non-rdflib parser (TURTLE_ITER, NT) is used. In the rest of cases, BNodes are ignored.
• No @ shape will appear in any constraint. The macro BNode is now properly used.
• The macro NONLITERAL has been introduced to generalize situations in which both BNodes and IRIs play the same role w.r.t some target nodes.
• Bug related to cardinality in OR constraints has been detected and corrected.