MJCF to SDF: Add serial joints

[ahcorde]

🎉 New feature

Summary

This PR allows to create joint in serial if there is the need

You can try this with the humanoid.xml world available in dm_control.

Issues:

• I change all geom to cylinders. Collisions are not working properly with capsules. Feet are not colliding with the floor
• If I used capsules I can't activate self_collide, but with cylinders I can.

Test it

Checklist

• [ ] Signed all commits for DCO
• [ ] Added tests
• [ ] Added example and/or tutorial
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [ ] 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 #62 (c3a63ec) into main (6a76012) will decrease coverage by 0.15%. The diff coverage is 96.10%.

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
- Coverage   95.20%   95.05%   -0.16%     
==========================================
  Files          24       24              
  Lines        1106     1152      +46     
==========================================
+ Hits         1053     1095      +42     
- Misses         53       57       +4     

Impacted Files	Coverage Δ
...sdformat_mjcf/mjcf_to_sdformat/converters/world.py	97.47% <95.94%> (-1.16%)	:arrow_down:
...sdformat_mjcf/mjcf_to_sdformat/converters/joint.py	89.23% <100.00%> (-3.08%)	:arrow_down:
.../sdformat_mjcf/mjcf_to_sdformat/converters/link.py	100.00% <100.00%> (ø)

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[azeey]

I just tested this with dm_control/dm_control/suite/cheetah.xml and it doesn't seem to be creating serial joints. I wonder if it doesn't handle joints whose parents are "world" properly.

[ahcorde]

I'm able to load the world but I have to remove these 3 joints

      <joint name="rootx" type="slide" axis="1 0 0" class="free"/>
      <joint name="rootz" type="slide" axis="0 0 1" class="free"/>
      <joint name="rooty" type="hinge" axis="0 1 0" class="free"/>

I'm not really sure why these joints are there, maybe to report the state ?