Independence posets

[fchapoton]

this is from #33659, with clean-up.

Note : related code can be found in https://github.com/haruhisa-enomoto/tors-lattice

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

[tscrim]

Still some more cleanup to do (e.g., _inc needs doctests, lots of one-line docstrings needed). I will try to do this soon. Although it seems like there is an additional class to be constructed.

Relevant paper: https://arxiv.org/abs/1805.00815

[fchapoton]

yes, absolutely not ready, and parts of that are already in sage, for instance is_trim. I just wanted to start cleaning.

[github-actions]

Documentation preview for this PR (built with commit 56c18da; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[dcoudert]

There is a lint error

[fchapoton]

there are still some one-line descriptions missing. Also one may like to have more explicit function names