Create a UNTL document in a string from a UNTL metadata
        root object.

        untl_xml_string = metadata_root_object.create_xml_string()

def create_xml_string(self):
        """Create a UNTL document in a string from a UNTL metadata
        root object.

        untl_xml_string = metadata_root_object.create_xml_string()
        """
        root = self.create_xml()

        xml = '<?xml version="1.0" encoding="UTF-8"?>\n' + tostring(
            root, pretty_print=True
        )
        return xml