Add nodiscard and example usage in Actor

[Ryanf55]

🎉 New feature

Relates to #1358

Summary

• Add a macro to start using nodiscard as recommended in 1358.
• Demonstrate usage of the macro in Actor.hh
• If this PR is the right approach, then a future PR (or PR's) can fold this across to the rest of the repo.

In theory, clang-tidy should be able to automatically apply this across the whole repo, but clang-tidy doesn't seem to work with this repo. If anyone knows how to use it, that would be much preferable because automated code refactors are much less error-prone and quicker than me doing it manually.

Test it

• Compilation is the only thing needed
• If you want, try calling a getter in an application, and ignoring the return value. If you compile with C++17, it should warn.
• Same as above, but if you compile with C++14, you should not expect a warning.

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.

[scpeters]

per #1358 (comment), we don't need to define this macro, we can use [[nodiscard]] directly in the header file. apologies for the back-and-forth

[Ryanf55]

No worries. While I think I could do this to the model, this is perhaps a great candidate for using clang-tidy's automated refactoring capabilities. Although I don't have experience with it, perhaps someone at Gazebo does?

I don't really expect to have time to do it to the entire repo, at least manually. It's not quick work because I don't think you can find-replace or regex it.

In the PR, you can see I tried using the modernize-use-nodiscard builtin, but I could not get it working correctly.

[scpeters]

I haven't configured clang-tidy before. I think it would be worth adding [[nodiscard]] to this class by hand and then comparing to what clang-tidy generates

[scpeters]

I installed clang-tidy on a debian machine and built Ionic up to sdformat15 in a colcon workspace and then was able to apply the [[nodiscard]] attribute automatically using the following command after sourcing the workspace:

clang-tidy --fix -checks=-*,modernize-use-nodiscard src/sdformat/include/sdf/*.hh --  $(pkg-config --cflags sdformat15)

[Ryanf55]

Fantastic. I'll go ahead and do the work manually and post a diff.

[scpeters]

untested changes to all header files in 1d96a58

I'm pretty sure it pushes some lines over 80 characters, which would need to be fixed

[azeey]

@Ryanf55, @scpeters we don't have much time left before the code freeze. Is this likely to make it by next Wednesday (08/28)?