[proposal] RDFLoader / RobotModelLoader: remove parsing of TinyXML documents

[rhaschke]

urdf::Model::initXml() is highly inefficient, as it first serializes the string and eventually
parses it again (see ros/urdf#24 (comment)). We should discourage and eventually remove the TinyXML API and solely rely on parsing from string.

For now, this is just a proof-of-concept that the MoveIt source repo doesn't need the TinyXML support.
A user-friendly approach would deprecate the TinyXML API first, but, maybe, that's not really necessary?

Opinions?

[jonbinney]

+1! I hadn't realized initXml() was doing that deserialize-reserialize step....

[v4hn]

I don't have a strong opinion here, but if we want to remove it in melodic, please add a line to the MIGRATION.md

[v4hn]

This code triggers a deprecation warning in the build farm now, because urdf::Model deprecated TinyXML.

So I guess this request is more about whether we want to transition to tinyxml2 or only remove the old API.

Given @rhaschke's argumentation, I agree to only remove the old API in melodic-devel.

[rhaschke]

Added the requested migration note. @v4hn, do I understand correctly, that you accept the removal without a deprecation phase?

[v4hn]

do I understand correctly, that you accept the removal without a deprecation phase

Yes. I believe it is unnecessary overhead to drag this along as deprecated.
If you agree, please merge.