Independence posets

[fchapoton]

this is from #33659, with clean-up.

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[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 8bf6574; changes) is ready! 🎉
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[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