URDF improvements

[clalancette]

@traversaro

Anyhow, the .xsd is already stored in urdfdom (see urdfdom/urdf.xsd at 41838e1f3623e0538a402024b76f296dbbcbed9f · ros/urdfdom · GitHub), so probably the easiest thing is indeed to also keep the specification there as well, and then to think about how to handle tagging later. We could do like for sdformat, in which there is the SDF version and the library version that are different, but they are mantained in the same repo.

Yes, this is exactly my thinking. The git tag on the repository is essentially tagging the code, while the version of URDF that is supported is in the specification and XSD.

My issue with splitting it up is that the set of URDF repositories is already highly fractured and hard to follow. Adding yet another repository won’t help with that situation. We can consider doing splitting it out in the future if we think that is worthwhile.