Add dual_annealing algorithm

[reneeotten]

Description

This PR adds the ``dual_annealingalgorithm that was introduced inscipy``` v1.2 (see Issue #527)

Type of Changes

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

Tested on

Python: 3.7.3 (default, Mar 31 2019, 14:30:14)
[Clang 10.0.1 (clang-1001.0.46.3)]

lmfit: 0.9.13+29.g63369d3, scipy: 1.2.1, numpy: 1.16.3, asteval: 0.9.13, uncertainties: 3.0.3, six: 1.12.0

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?
• squashed/minimized your commits and written descriptive commit messages?
• added or updated existing tests to cover the changes?
• updated the documentation?
• added an example?

[codecov]

Codecov Report

Merging #556 into master will increase coverage by 0.03%.
The diff coverage is 88.57%.

@@            Coverage Diff             @@
##           master     #556      +/-   ##
==========================================
+ Coverage   84.68%   84.72%   +0.03%     
==========================================
  Files          11       11              
  Lines        3213     3247      +34     
==========================================
+ Hits         2721     2751      +30     
- Misses        492      496       +4

Impacted Files	Coverage Δ
lmfit/minimizer.py	90.14% <88.57%> (-0.09%)	⬇️

---

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[codecov]

Codecov Report

Merging #556 into master will increase coverage by 0.03%.
The diff coverage is 88.57%.

@@            Coverage Diff             @@
##           master     #556      +/-   ##
==========================================
+ Coverage   84.68%   84.72%   +0.03%     
==========================================
  Files          11       11              
  Lines        3213     3247      +34     
==========================================
+ Hits         2721     2751      +30     
- Misses        492      496       +4

Impacted Files	Coverage Δ
lmfit/minimizer.py	90.14% <88.57%> (-0.09%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 76537d1...63369d3. Read the comment docs.

[newville]

should these two imports be grouped together? If either import fails, neither will be available. I thought they were introduced in different versions of scipy.

[reneeotten]

both algorithms were added in scipy v1.2.0 so, yes, the grouping is intentional

[newville]

Oh, OK. Thanks!

[newville]

@reneeotten Looks Great! I had one small comment/question, but let me know if/when you think this is ready to merge.

[newville]

@reneeotten Let me know if this is ready to merge... Looks great to me!

[reneeotten]

it should be good to go as far as I am concerned so I'll merge it.