Allow model frames (__model__) to be used as joint parent or child

[azeey]

🦟 Bug fix

Summary

The spec (http://sdformat.org/tutorials?tut=composition_proposal&cat=pose_semantics_docs&#1-3-3-model-frame-references) says "For a model named {name}, model frames can be referenced by their name. {name}::__model__ is also valid, but {name} is preferred instead." libsdformat has supported using a model's {name} in //joint/parent and //joint/child for a while, but it currently does not support {name}::__model__. This PR rectifies that and also adds support for using just __model__ since that is a valid frame inside a model.

Came up in PR review for #768: comment

Checklist

• Signed all commits for DCO
• Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[scpeters]

Ubuntu cmake warning:

CMake Warning:
  Manually-specified variables were not used by the project:

    SKIP_usd

[scpeters]

Ubuntu cmake warning:

CMake Warning:
  Manually-specified variables were not used by the project:

    SKIP_usd

related to gazebo-tooling/release-tools#625

[scpeters]

Ubuntu cmake warning:

related to ignition-tooling/release-tools#625

fix in gazebo-tooling/release-tools#632

[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-03-25-fortress-edifice-citadel/1343/1

[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-04-13-fortress-edifice/1367/1