Use MJCF default values for gravity, magnetic, and wind when not set

[azeey]

🦟 Bug fix

Summary

We weren't using the correct MJCF default values for gravity, magnetic, and wind. The default for wind matches the SDFormat default so it wasn't an issue, but the other elements, especially, magnetic, had different values.

Checklist

• 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)
• 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]

Codecov Report

Merging #109 (0b793b4) into main (f644622) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #109   +/-   ##
=======================================
  Coverage   95.20%   95.20%           
=======================================
  Files          24       24           
  Lines        1168     1168           
=======================================
  Hits         1112     1112           
  Misses         56       56           

Impacted Files	Coverage Δ
...sdformat_mjcf/mjcf_to_sdformat/converters/world.py	97.56% <100.00%> (ø)

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