Add ultranest sampler (Sourcery refactored)

[sourcery-ai]

Pull Request #246 refactored by Sourcery.

If you're happy with these changes, merge this Pull Request using the Squash and merge strategy.

NOTE: As code is pushed to the original Pull Request, Sourcery will
re-run and update (force-push) this Pull Request with new refactorings as
necessary. If Sourcery finds no refactorings at any point, this Pull Request
will be closed automatically.

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To manually merge these changes, make sure you're on the ultranest branch, then run:

git fetch origin sourcery/ultranest
git merge --ff-only FETCH_HEAD
git reset HEAD^

Help us improve this pull request!

[sourcery-ai]

Function PymultinestTest.test refactored with the following changes:

• Applies the dictionary union operator where applicable (use-dictionary-union)

This removes the following comments ( why? ):

# noqa