enable jacobian argument for optimization

[jgabry]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Adds jacobian argument to optimize method.
Closes #761 and paves the way for #760 (adding Laplace sample)

Copyright and Licensing

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(this will be you or your assignee, such as a university or company):
Columbia University

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[jgabry]

I think this is ready for review. I put both @rok-cesnovar and @andrjohns as reviewers but this probably only needs one of you, it's not too complicated.

[codecov-commenter]

Codecov Report

Merging #799 (37878f5) into master (407db8c) will decrease coverage by 0.03%.
The diff coverage is 90.00%.

❗ Current head 37878f5 differs from pull request most recent head b47a327. Consider uploading reports for the commit b47a327 to get more accurate results

@@            Coverage Diff             @@
##           master     #799      +/-   ##
==========================================
- Coverage   88.27%   88.24%   -0.03%     
==========================================
  Files          12       12              
  Lines        4152     4160       +8     
==========================================
+ Hits         3665     3671       +6     
- Misses        487      489       +2     

Files Changed	Coverage Δ
R/args.R	97.76% <88.88%> (-0.16%)	⬇️
R/model.R	90.59% <100.00%> (+0.01%)	⬆️

... and 1 file with indirect coverage changes

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[jgabry]

I'll work on #760 next

[andrjohns]

LGTM, thanks!

[jgabry]

LGTM, thanks!

Thanks for reviewing. Will merge now and continue working on the Laplace method in #800