add chi2_out and nsigma options to conf_interval2d

[newville]

Description

This adds chi2_out and nsigma parameters to conf_interval2d, and adds documentation and an example of using it to make contour maps from a matrix of chi-square values.

This works to clarify some of the discussion at #848

Type of Changes

• Bug fix
• New feature
• Refactoring / maintenance
• Documentation / examples

Tested on

Python: 3.9.15 (main, Nov 24 2022, 08:29:02)
[Clang 14.0.6 ]

lmfit: 1.1.0.post9+gd2ba8e9.d20230312, scipy: 1.9.3, numpy: 1.23.5, asteval: 0.9.29, uncertainties: 3.1.7

Verification

Have you

• included docstrings that follow PEP 257?

• referenced existing Issue and/or provided relevant link to mailing list?

• verified that existing tests pass locally?

• verified that the documentation builds locally?

• squashed/minimized your commits and written descriptive commit messages?

• added or updated existing tests to cover the changes?
• updated the documentation and/or added an entry to the release notes (doc/whatsnew.rst)?
• added an example?

[codecov]

Codecov Report

Merging #852 (e84733c) into master (d2ba8e9) will decrease coverage by 0.03%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #852      +/-   ##
==========================================
- Coverage   93.75%   93.72%   -0.03%     
==========================================
  Files          10       10              
  Lines        3520     3522       +2     
==========================================
+ Hits         3300     3301       +1     
- Misses        220      221       +1     

Impacted Files	Coverage Δ
lmfit/confidence.py	97.19% <80.00%> (-0.45%)	⬇️

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[reneeotten]

thanks @newville - this is a nice enhancement with a good example on how to use it and I'd say almost ready to merge! I've left a few tiny comments for your consideration.

I am going through the other PR - sorry it takes me so long.... I might try to clean-up the commit history there a bit to group related commits together so it's easier to follow. Of course, if you want that PR already without any clean-up -- I know. you don't mind a bit of mess too much ;) -- that's fine with me as well.

[newville]

@reneeotten thanks very much, I made all suggested changes. And, yeah, I know that #844 is slightly enormous and started to grow several not-exactly-related changes. I'm definitely OK with merging this and #853 before #844.