Starting to develop SDF stuff
Fixed bug in Robot.add_joint() (thanks to Carlos De La Guardia). Fixed some SDF stuff
Merge branch 'feature/sdf' into reflection. SDF stuff incomplete, but thought I would keep it all together.
Renamed to_xml to dump_xml, but that is inconsistent with pyyaml. Will rename to {read,write}_xml for clarity.
Finished refactoring. Instance methods: to_xml => write_xml, load_xml => read_xml. Class methods: from_{xml,string}