Buoyancy: fix center of volume's reference frame

[chapulina]

🦟 Bug fix

Summary

While working with a model that uses NED / FSK orientation, I noticed that the center of volume calculation on the buoyancy plugin was using the wrong reference frame. According to the documentation and the usage, the reference frame is expected to be the parent link. But the value provided was being calculated in the world frame. That's evident when using a model that's oriented with Z down.

Test it

I added a test world with 4 models that are supposed to be neutrally buoyant. Without this fix, the model that has a CoV offset and starts with a non-zero orientation isn't stable:

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)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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[codecov]

Codecov Report

Merging #1302 (0bb551d) into ign-gazebo3 (6f58c91) will decrease coverage by 0.08%.
The diff coverage is 71.01%.

❗ Current head 0bb551d differs from pull request most recent head 134056a. Consider uploading reports for the commit 134056a to get more accurate results

@@               Coverage Diff               @@
##           ign-gazebo3    #1302      +/-   ##
===============================================
- Coverage        77.25%   77.16%   -0.09%     
===============================================
  Files              227      227              
  Lines            13535    13574      +39     
===============================================
+ Hits             10457    10475      +18     
- Misses            3078     3099      +21     

Impacted Files	Coverage Δ
...clude/ignition/gazebo/components/CenterOfVolume.hh	100.00% <ø> (ø)
src/systems/air_pressure/AirPressure.cc	72.83% <69.23%> (-1.14%)	⬇️
src/systems/altimeter/Altimeter.cc	73.49% <69.23%> (-1.18%)	⬇️
src/systems/logical_camera/LogicalCamera.cc	75.00% <69.23%> (-1.25%)	⬇️
src/systems/magnetometer/Magnetometer.cc	70.78% <69.23%> (-0.82%)	⬇️
src/systems/imu/Imu.cc	71.42% <71.42%> (-0.87%)	⬇️
src/systems/buoyancy/Buoyancy.cc	75.00% <100.00%> (-0.29%)	⬇️
src/SimulationRunner.cc	93.45% <0.00%> (-1.07%)	⬇️

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[arjo129]

Thanks for catching this. Tested before and after the fix and can confirm this fix indeed does solve the problem at hand. Same problem affects graded buoyancy ☠️ I'll go take alook and fix it.

[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-2022-03-01-citadel-edifice-fortress/1313/1

[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