migrate non-standard URDF `capsule` tag to `drake:capsule`.

[rpoyner-tri]

(Re-)discovrered while writing parsing docs. We support some non-standard legacy capsule tag in URDF, but the standard is unaware of it. Proposal: support drake:capsule, and maybe deprecate the made-up capsule tag. If URDF ever introduces capsule, at least we (hopefully by then) won't have old tags with potentially clashing syntax and semantics lying about.

FYI @EricCousineau-TRI @DamrongGuoy @joemasterjohn thoughts?

[jwnimmer-tri]

Counter-proposal: add <capsule> to the URDF spec using our current semantics, and change nothing in Drake.

[jwnimmer-tri]

Here is the TODO in the current code:

drake/multibody/parsing/detail_urdf_geometry.cc

Lines 291 to 304 in 504c6f1

	} else if (child_node = node->FirstChildElement("capsule");
	child_node) {
	// TODO(SeanCurtis-TRI): This should *not* be <capsule>. It should be
	// <drake:capsule>. <capsule> is not in the spec
	// http://wiki.ros.org/urdf/XML/link. And even there has been a three-year
	// debate about adding it into ros (still unresolved):
	// https://github.com/ros/urdfdom_headers/pull/24
	// Given that this is a tag that is *not* in the spec, it requires the
	// namespace.
	// As a footnote, bullet does support it:
	// https://github.com/bulletphysics/bullet3/blob/master/data/capsule.urdf
	// and we have a number of legacy files that have <capsule> declarations
	// in them.
	return ParseCapsule(child_node);

It mentions ros/urdfdom_headers#24.

[rpoyner-tri]

I read the TODO; that's how I got here. The consequences of your counter-proposal are: (1) delete the TODO, (2) document some set of non-prefixed community extensions to URDF that Drake honors, (3) maybe delete that documentation if the standard version ever merges.

[jwnimmer-tri]

Yes, that sounds correct to me. I think our users are best served by keeping our current (quite reasonable) extension intact.

While the thoughts captured in the TODO make sense from a conformance perspective, I don't think they actually help users in any way. If we had started with <drake:capsule> from day 1, perhaps there would be a case for keeping it, but rolling it back now doesn't help anyone.

[rpoyner-tri]

Got it. I think I'll close the issue, and incorporate items 1 and 2 from above into #16444.