feat: least-squares iterator

[maxdinkel]

Description and Context

This PR replaces the old LMIterator implementation with a more general LeastSquaresIterator based on the SciPy implementation. This allows one to choose between three algorithms (lm, dogbox, trf) for optimizing a least squares objective.

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How Has This Been Tested?

I removed the unit tests for the old LMIterator, but adapted the existing integration tests to test all algorithms (lm, dogbox, trf).

Checklist

• My commit messages mention the appropriate issue numbers (advised but optional).
• I mention the appropriate issue numbers in this PR.
• I updated documentation where necessary.
• I have added tests to cover my changes.

Additional Information

Interested Parties

@sbrandstaeter

Possible reviewers:

Other interested parties:

[gilrrei]

lgtm

[leahaeusel]

Looks good, thanks for updating this iterator! I just have two minor comments.