Support adding systems that don't come from a plugin

[chapulina]

🎉 New feature

Summary

This is broken off #926 for easier review.

When using the Server API directly, it's often useful to instantiate systems locally without actually loading a plugin's shared library. This PR adds a new Server::AddSystem that takes a raw pointer and behaves just like the existing AddSystem function that takes a plugin.

I changed the test Relay class to use this approach, which should be a bit faster than loading the shared library.

Also added the convenient gazebo::worldEntity function that returns the first world found.

Test it

Take a look at the unit tests added, and see on #926 how the test fixture uses this.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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

🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸

[codecov]

Codecov Report

Merging #936 (aabd086) into ign-gazebo3 (eae6b9c) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           ign-gazebo3     #936      +/-   ##
===============================================
+ Coverage        77.78%   77.83%   +0.05%     
===============================================
  Files              219      219              
  Lines            12566    12593      +27     
===============================================
+ Hits              9774     9802      +28     
+ Misses            2792     2791       -1     

Impacted Files	Coverage Δ
include/ignition/gazebo/Server.hh	100.00% <ø> (ø)
src/Server.cc	84.33% <100.00%> (+1.53%)	⬆️
src/SimulationRunner.cc	94.49% <100.00%> (+0.06%)	⬆️
src/SimulationRunner.hh	100.00% <100.00%> (ø)
src/Util.cc	93.03% <100.00%> (+0.07%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update eae6b9c...aabd086. Read the comment docs.

[azeey]

Thanks for the addition. This will be really useful. I have a couple of comments about the API.

[mjcarroll]

Great addition! Do you think we should add some documentation/tutorial for developers so that they are aware that this option exists? (Could be done in a follow-up, not going to block this)

[azeey]

I see UNIT_ModelCommandAPI_TEST failed on macOS. Is that a new flake?

[mjcarroll]

I see UNIT_ModelCommandAPI_TEST failed on macOS. Is that a new flake?

It's a new test I believe from the ign model command?

[chapulina]

Do you think we should add some documentation/tutorial for developers so that they are aware that this option exists?

I could write up a simple tutorial, but I'm not sure in which context we want to encourage users to use this API directly. The TestFixture class added in #926 uses it internally, and I wrote a tutorial/example using that class because I can see a clear use case.