Apply gravity external to dartsim for added mass

[mjcarroll]

🎉 New feature

Summary

When fluid added mass (#384) is used, dartsim will apply gravitational force to the total virtual mass of each link. Since this mass is only to model the inertial aspects of the dynamics, this is incorrect behavior.

In this PR, I disable dart's application of gravity to links that have added mass defined in their sdf file. Instead, gravity is applied as an external force right before the world update by multiplying the link mass by the gravity vector (F = ma!).

Test it

The unit test verifies that when added mass is set to all zeros, that gravity continues to work as expected.

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]

Codecov Report

Merging #462 (54deeb8) into gz-physics6 (70059ae) will increase coverage by 0.09%.
The diff coverage is 94.64%.

@@               Coverage Diff               @@
##           gz-physics6     #462      +/-   ##
===============================================
+ Coverage        75.43%   75.52%   +0.09%     
===============================================
  Files              140      142       +2     
  Lines             7099     7138      +39     
===============================================
+ Hits              5355     5391      +36     
- Misses            1744     1747       +3     

Impacted Files	Coverage Δ
dartsim/src/plugin.cc	100.00% <ø> (ø)
dartsim/src/SDFFeatures.cc	62.66% <57.14%> (-1.42%)	⬇️
dartsim/src/AddedMassFeatures.cc	100.00% <100.00%> (ø)
dartsim/src/SimulationFeatures.cc	95.00% <100.00%> (+0.55%)	⬆️
include/gz/physics/AddedMass.hh	100.00% <100.00%> (ø)

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[JoanAguilar]

The implementation looks good, my major comment is for the unit test, which only checks gravity application for a body with zero added mass.

[mabelzhang]

I've read the code, but I'm not familiar with gz-physics, so I'm leaning on Joan's review for the most part.

For the tests, I guess the top-level test is test/common_test/added_mass.cc, and it's looking at 3 spheres without added mass, and 1 sphere with added mass. I don't know enough to say whether that's sufficient, but more tests can always be added later.

Since this is a new feature and doesn't break existing things, and CI is passing, it's probably okay.