Remove gear3 integrator

[blaisb]

Description

The Gear3 integrator was not used anywhere in the code in any example or in any tests. This integrator never gave good performance and was actually not tested for years. I believe it is now time to fully delete it. I want to work on adding new integrators this year and this will require some more unification. I want to start by cleaning things before adding new things.

Testing

There were actually no test for gear3...

Documentation

I adapted the documentation but the gear3 integrator was never mentioned in the documentation.

Miscellaneous (will be removed when merged)

Checklist (will be removed when merged)

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Code related list:

• All in-code documentation related to this PR is up to date (Doxygen format)
• Lethe documentation is up to date
• The branch is rebased onto master
• Code is indented with indent-all and .prm files (examples and tests) with prm-indent
• If parameters are modified, the tests and the documentation of examples are up to date
• Changelog (CHANGELOG.md) is up to date if the refactor affects the user experience or the codebase

Pull request related list:

• No other PR is open related to this refactoring
• Labels are applied
• There are at least 2 reviewers (or 1 if small feature) excluding the responsible for the merge
• If this PR closes an issue or is related to a project, it is linked in the "Projects" or "Development" section
• If any future works is planed, an issue is opened
• The PR description is cleaned and ready for merge