Change link description to use choice instead of all.

[dljsjr]

See #95

Unbounded elements in all are fine in XSD 1.1 but not valid in XSD 1.0.
There aren't a ton of technologies out in the wild that leverage XSD 1.1
(e.g. code generators). This definition should be acceptable for
validation of XML documents but I'm not convinced it's a nice way to do
things re: code generation, as the code generated by XJC for example
contains a single List field containing the inertial and the
collisions/visuals.

[scpeters]

@clalancette @sloretz do you know if ROS uses this schema file? I'd like to test and merge this

[sloretz]

@scpeters I looked around and didn't find anything using this schema file. Looking around this repo, it appears urdf.xsd isn't installed. Someone using it would have had to copy it from source. Edit I'm not sure how this is packaged but urdfdom-0.4.1 has the schema in its archive http://archive.ubuntu.com/ubuntu/pool/universe/u/urdfdom/urdfdom_0.4.1.orig.tar.gz

Interestingly there are a couple examples of people writing their own schema.

• https://github.com/HBPNeurorobotics/BlenderRobotDesigner/blob/master/resources/urdf.xsd
• https://github.com/laas/urdf_validator/blob/master/urdf.xsd

One uses <all> but limits <visual> and <collision> to 1 occurrence. The other uses a sequence and forces children of <link> to be ordered.

[aminya]

The most up-to-date urdf.xsd seems to be this file:
https://github.com/RobotLocomotion/drake/blob/7c24d00af82587dd717bdb1152c885a78f68a3b3/doc%2FdrakeURDF.xsd