[Citadel] [TPE] Add velocity cmd

[claireyywang]

This PR is consisted of a few parts

1. creates new AngularVelocityCmd and LinearVelocityCmd components
2. adds new optional WorldVelocityCmdFeatureList for SetFreeGroupWorldVelocity feature
3. adds functions that updates angular and linear velocities of Free Group entity in Physics.cc
4. creates a new demo world velocity_demo.sdf, which is basically diff_drive.sdf but with different names. I created a separate sdf file to avoid confusion, but open to just use diff_drive.sdf.
5. creates new systems plugin named velocity_demo that generates angular/linearVelocityCmd on model.

Use the following cmds for testing

$ ign gazebo -v 2 velocity_demo.sdf --physics-engine ignition-physics-tpe-plugin

# in a separate terminal window
$ ign topic -t "/model/vehicle_green/cmd_vel" -m ignition.msgs.Twist -p "linear: {x: 1.0}, angular: {z: 0.1}"
$ ign topic -t "/model/vehicle_blue/cmd_vel" -m ignition.msgs.Twist -p "linear: {x: 0.5}, angular: {z: 0.05}"

then hit play button on ignition gazebo window.

Demo gif see below

[codecov]

Codecov Report

Merging #169 into ign-gazebo3 will decrease coverage by 0.39%.
The diff coverage is 15.78%.

@@               Coverage Diff               @@
##           ign-gazebo3     #169      +/-   ##
===============================================
- Coverage        65.33%   64.93%   -0.40%     
===============================================
  Files              127      129       +2     
  Lines             6311     6349      +38     
===============================================
  Hits              4123     4123              
- Misses            2188     2226      +38     

Impacted Files	Coverage Δ
src/systems/physics/Physics.cc	32.58% <5.88%> (-1.26%)	⬇️
...e/ignition/gazebo/components/AngularVelocityCmd.hh	100.00% <100.00%> (ø)
...de/ignition/gazebo/components/LinearVelocityCmd.hh	100.00% <100.00%> (ø)
src/SimulationRunner.cc	86.36% <0.00%> (-1.19%)	⬇️

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[claireyywang]

[ignore]

[iche033]

thanks for adding the integration test. I expanded it a little to check that the robot is moving in the right direciton: 1b88b37

Just left another minor comment. Otherwise changes look good to me.

[claireyywang]

@osrf-jenkins run tests please

[claireyywang]

@iche033 Thanks for adding the additional test! I removed the unused Command struct in f9064e9. The homebrew CI failures and warnings don't seem to be related to this PR. Ready for another look whenever you are, thanks!

[iche033]

looks good to me!

[claireyywang]

@osrf-jenkins run tests please

[jacobperron]

It looks like this change introduced test failures to ignition_gazebo-ci-ign-gazebo3-homebrew-amd64:

• INTEGRATION_physics_system/test_ran (added as new flake)
• PeerTracker/PeerTrackerStale (known flake)

[jacobperron]

It looks like the PeerTracker failure is a known flake (#8), but I'm not sure about the INTEGRATION_physics_system test.

[claireyywang]

It looks like the PeerTracker failure is a known flake (#8), but I'm not sure about the INTEGRATION_physics_system test.

Im trying to reproduce the physics_system test fail on my local. I'll update here when I get a result.

[claireyywang]

weird, I wasn't to get the same test fail on my local build. I reran the homebrew test to see if the failure pops up again

[claireyywang]

@jacobperron INTEGRATION_physics_system seems to be another flaky test.