code: def set_context(self):
          
          results = self.rml.query("""
                  SELECT ?o {
                      {
                          ?s rr:class ?o
                      } UNION {
                          ?s rr:predicate ?o
                      }
                  }""")
          namespaces = [Uri(row[0]).value[0]
                        for row in results
                        if isinstance(row[0], rdflib.URIRef)]
          self.context = {ns[0]: ns[1] for ns in namespaces if ns[0]}
docs: Reads throught the namespaces in the RML and generates a context for
          json+ld output when compared to the RdfNsManager namespaces