returns the rml mapping RdfDataset

        rml_name(str): Name of the rml mapping to retrieve

def get_rml(self, rml_name):
        """ returns the rml mapping RdfDataset

        rml_name(str): Name of the rml mapping to retrieve
        """

        try:
            return getattr(self, rml_name)
        except AttributeError:
            return self.load_rml(rml_name)