mjcf_to_sdformat: set //model/self_collide to true

[scpeters]

🦟 Bug fix

Allows contact between links after converting from MJCF -> SDFormat

Summary

When converting from MJCF to SDFormat, all links are placed in the same model. With this arrangement, no contacts are possible unless //model/self_collide is set to True. Since it defaults to False, set it to True explicitly.

Checklist

• [X] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [X] codecheck passed (See contributing)
• [X] 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 #73 (78e4ad0) into main (7d75e16) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #73   +/-   ##
=======================================
  Coverage   97.22%   97.22%           
=======================================
  Files          19       19           
  Lines         828      829    +1     
=======================================
+ Hits          805      806    +1     
  Misses         23       23           

Impacted Files	Coverage Δ
...f_to_sdformat/mjcf_to_sdformat/converters/world.py	98.43% <100.00%> (+0.02%)	:arrow_up:

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