Buoyancy

[nkoenig]

Redo of the buoyancy PR. Original is here.

This will resolve issue #159.

[codecov]

Codecov Report

Merging #252 into ign-gazebo3 will decrease coverage by 0.75%.
The diff coverage is 24.63%.

@@               Coverage Diff               @@
##           ign-gazebo3     #252      +/-   ##
===============================================
- Coverage        65.71%   64.95%   -0.76%     
===============================================
  Files              127      131       +4     
  Lines             6238     6351     +113     
===============================================
+ Hits              4099     4125      +26     
- Misses            2139     2226      +87     

Impacted Files	Coverage Δ
src/systems/user_commands/UserCommands.hh	100.00% <ø> (ø)
src/systems/physics/Physics.cc	32.58% <16.07%> (-2.67%)	⬇️
src/systems/user_commands/UserCommands.cc	11.85% <16.66%> (+0.23%)	⬆️
...e/ignition/gazebo/components/AngularVelocityCmd.hh	100.00% <100.00%> (ø)
...clude/ignition/gazebo/components/CenterOfVolume.hh	100.00% <100.00%> (ø)
...de/ignition/gazebo/components/LinearVelocityCmd.hh	100.00% <100.00%> (ø)
include/ignition/gazebo/components/Volume.hh	100.00% <100.00%> (ø)
src/LevelManager.cc	86.22% <100.00%> (ø)
src/SdfGenerator.cc	95.83% <100.00%> (+0.07%)	⬆️
src/systems/physics/Physics.hh	100.00% <100.00%> (+90.90%)	⬆️
... and 6 more

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[chapulina]

LGTM. I think the only unaddressed comment from the original PR is this one from @azeey :

---

@azeey

"It would be good to document the reference frame of the position."

@nkoenig

"What language would you recommend? There is no frame attached to this component, so a system can use what ever frame they desire."

@azeey

"That's true, but I think that would lead to confusion. I would recommend maybe saying that the position is relative to the pose of the entity this component is attached to. If we need to specify it in world frame, we can create a CenterOfVolumeInWorld component like we did for Pose and WorldPose. "

---

Also, I'd appreciate it if you could also take another look, @azeey .

[nkoenig]

LGTM. I think the only unaddressed comment from the original PR is this one from @azeey :

@azeey

"It would be good to document the reference frame of the position."

@nkoenig

"What language would you recommend? There is no frame attached to this component, so a system can use what ever frame they desire."

@azeey

"That's true, but I think that would lead to confusion. I would recommend maybe saying that the position is relative to the pose of the entity this component is attached to. If we need to specify it in world frame, we can create a CenterOfVolumeInWorld component like we did for Pose and WorldPose. "

Also, I'd appreciate it if you could also take another look, @azeey .

Add position documentation in 9eda1a7