Simple, raw XML value. Need to bugfix putting this back into a document
If type has 'write_xml', this function should expect to have it's own XML already created i.e., In Axis.to_sdf(self, node), 'node' would be the 'axis' element. @todo Add function that makes an XML node completely independently?
Reimplemented from urdf_parser_py.xml_reflection.core.ValueType.