Allow for more complex joints in URDF

[traversaro]

@scastro

For example, URDF joint limits only provide position, velocity, and effort limits, and there have been a whole bunch of discussions in the MoveIt space on adding acceleration limits to the allowable set of tags. This has gone nowhere due to the “slippery slope” argument: that is, if we add acceleration limits, then someone is going to want jerk limits next, etc. There is validity to this, as you will certainly encounter scope creep from implementing every such request.

Just FYI, I think part of the problem related to modifying the URDF spec is that currently there is no version control (i.e. git) or versioning (i.e. git tags) of the spec itself, so it is not possible to discuss changes in the spec itself.

This was discussed in Move URDF specification to a Github repo? · Issue #62 · ros/urdfdom_headers · GitHub and URDF improvements - #17 by traversaro . I started working on it in https://github.com/ros/urdfdom/pull/183 but unfortunately I run out of time on working on this back in time, but if anyone wants to continue that work and complete the PR I think it would drastically simplify the discussion of possible changes of the URDF spec.