creates a namespace if it does not exist

        args:
            name: the name of the namespace
            ignore_errors(bool): Will ignore if a namespace already exists or
                    there is an error creating the namespace

        returns:
            True if created
            False if not created
            error if namespace already exists

def create_namespace(self, name, ignore_errors=False):
        """creates a namespace if it does not exist

        args:
            name: the name of the namespace
            ignore_errors(bool): Will ignore if a namespace already exists or
                    there is an error creating the namespace

        returns:
            True if created
            False if not created
            error if namespace already exists
        """
        if not self.has_namespace(name):
            self.namespaces[name] = ConjunctiveGraph()
            return True
        elif ignore_errors:
            return True
        else:
            raise RuntimeError("namespace '%s' already exists" % name)