Ensure FPS/CUR selected idxs are unique in the case of zero-score

[jwa7]

Attempting to fix #206

Hello!

I have encountered the issue raised by Alex in the above issue, whilst working with the equisolve wrapper for TensorMap-based sample/feature selection (i.e. in https://github.com/lab-cosmo/equisolve/blob/main/src/equisolve/numpy/sample_selection.py and co).

I have adapted Alex's example into a few unit tests for both FPS and CUR sample/feature selection, and attempted to fix it. However, there is something I'm not understanding. While the FPS tests now pass, there are a couple of (different) CUR ones that do not.

With this PR I was hoping to get some feedback/help from the skmatter dev team on this. Thanks! :)

Contributor (creator of PR) checklist

• [x] Tests updated (for new features and bugfixes)?
• [ ] ~Documentation updated (for new features)?~
• [x] Issue referenced (for PRs that solve an issue)?

For Reviewer

• [ ] CHANGELOG updated if important change?

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📚 Documentation preview 📚: https://scikit-matter--224.org.readthedocs.build/en/224/

[rosecers]

Are you still looking for feedback on this? Good measure to tag one of us for input.

[jwa7]

Hey! This isn't something I'm actively working on at the moment, but I'll be sure to ping you for feedback when I / we get back round to it :)