Fix graded buoyancy problems

[arjo129]

🦟 Bug fix

Fixes #1296 and #1210

Summary

The primary target of this PR is #1296. This PR changes an incorrect sign line 224 of the buoyancy plugin when it was running in graded buoyancy mode. Essentially when running the example in #1296 now both graded and uniform mode are unstable. This is the correct behavior. For a stable example see the added tests in this PR. In particular both SDFs should be stable and have small oscillations, but previously the graded buoyancy example would have kept on spinning.

Additionally #1210 would mean the graded buoyancy test would keep sinking. This PR removes the need for checking the newPose as we nolonger need the inertial pose. The reason I was initially using the inertialpose was because I had thought that Link::AddWorldWrench applies a force at the center of mass, but in reality the original behaviour in uniform buoyancy was correct as Link::AddWorldWrench applies a force at the link itself.

TL;DR

Before times when running https://github.com/ignitionrobotics/ign-gazebo/blob/1d461b849b92104299bc03879bb67db368fe10c9/test/worlds/buoyancy_graded_restoring_moments.sdf :

After times:

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

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 #1297 (987eca7) into ign-gazebo6 (05b9232) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           ign-gazebo6    #1297      +/-   ##
===============================================
- Coverage        62.25%   62.25%   -0.01%     
===============================================
  Files              278      278              
  Lines            23208    23203       -5     
===============================================
- Hits             14449    14444       -5     
  Misses            8759     8759              

Impacted Files	Coverage Δ
src/systems/buoyancy/Buoyancy.hh	100.00% <ø> (ø)
src/systems/buoyancy/Buoyancy.cc	82.60% <100.00%> (-0.42%)	⬇️

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[chapulina]

I think there may still be some offset calculations that are incorrect, see

• Documentation and more examples for #1297 #1301

[chapulina]

Besides #1301, I just have some more minor comments here.

[chapulina]

I pushed d013573 to disable the buoyancy engine test that was introduced on #1298 on Windows. It's not directly related to this PR, but it's also "graded buoyancy", so I'm taking advantage of the CI run here.

[chapulina]

LGTM with clean CI. I think we should eventually look deeper into why the 2 servers on the test are giving slightly different results though.

[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