broadcaster for magnetic field values from a magnetometer

[christianrauch]

This PR adds a broadcaster for magnetometers via the semantic_components::MagneticFieldSensor, similar to how this is done for other semantic components (semantic_components::IMUSensor, semantic_components::ForceTorqueSensor).

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• Fork the repository.
• Modify the source; please focus on the specific change you are contributing. If you also reformat all the code, it will be hard for us to focus on your change.
• Ensure local tests pass. (colcon test and pre-commit run (requires you to install pre-commit by pip3 install pre-commit)
• Commit to your fork using clear commit messages.
• Send a pull request, answering any default questions in the pull request interface.
• Pay attention to any automated CI failures reported in the pull request, and stay involved in the conversation.

[thedevmystic]

Hey @christianrauch, I see its some usage. Let's see what maintainers have to say.

And another thing, why is the class declaration is in implementation file (everything is in .cpp file).

It'll be really better if declaration was in .hpp and implementation was in .cpp

[bmagyar]

The hpp/cpp comment is a good catch indeed but since this is delivered as a plugin I'm not too concerned about it. Thanks for pointing it out @thedevmystic

[christianrauch]

I addressed the one comment about the maintainers and fixed one CI issue from my side.

The CI shows a couple of other issues, such as missing dependencies on controller_manager and backward_ros, which I suspect come from other packages since the magnetometer_broadcaster is not using controller_manager and backward_ros is already defined.