SDFormat to MJCF: Handle static models

[azeey]

🎉 New feature

Summary

Bodies converted from Links in static models are connected to the worldbody by a static joint.

This also introduces two new classes FreeJoint and StaticFixedJoint which can be used to designate joints that don't actually exist in the SDFormat file but are created by the converter to facilitate conversion to MJCF.

Test it

test_add_model.py

Checklist

• [x] Signed all commits for DCO
• [x] Added tests
• [ ] Added example and/or tutorial
• [x] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [x] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov[bot]]

Codecov Report

Merging #53 (e21095c) into main (06bcde4) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
+ Coverage   97.73%   97.77%   +0.03%     
==========================================
  Files          16       16              
  Lines         617      628      +11     
==========================================
+ Hits          603      614      +11     
  Misses         14       14              

Impacted Files	Coverage Δ
...ormat_to_mjcf/sdformat_to_mjcf/converters/joint.py	97.56% <100.00%> (+0.12%)	:arrow_up:
...ormat_to_mjcf/sdformat_to_mjcf/converters/model.py	100.00% <100.00%> (ø)
...dformat_to_mjcf/sdformat_to_mjcf/sdf_kinematics.py	95.55% <100.00%> (+0.81%)	:arrow_up:

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