Complex eval uncertainty

[newville]

Description

This gives fixes for #900 and fo #875

Tests are added for each.

Type of Changes

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

Tested on

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 #901 (e34b167) into master (875c2f7) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #901      +/-   ##
==========================================
- Coverage   93.25%   93.23%   -0.02%     
==========================================
  Files          10       10              
  Lines        3679     3683       +4     
==========================================
+ Hits         3431     3434       +3     
- Misses        248      249       +1     

Impacted Files	Coverage Δ
lmfit/model.py	91.25% <100.00%> (+0.04%)	⬆️

... and 1 file with indirect coverage changes