Reads throught the namespaces in the RML and generates a context for
        json+ld output when compared to the RdfNsManager namespaces

def set_context(self):
        """
        Reads throught the namespaces in the RML and generates a context for
        json+ld output when compared to the RdfNsManager namespaces
        """
        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]}