Add moment-matching support to $loo() method

[andrjohns]

Submission Checklist

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

Summary

This PR adds an option to the $loo() method: moment_match, which enables the use of a moment-matching correction for any problematic observations: https://mc-stan.org/loo/articles/loo2-moment-matching.html

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Andrew Johnson

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/)

[codecov-commenter]

Codecov Report

Merging #778 (e2a686e) into master (83bbc35) will decrease coverage by 0.92%.
The diff coverage is 100.00%.

❗ Current head e2a686e differs from pull request most recent head a6eb030. Consider uploading reports for the commit a6eb030 to get more accurate results

@@            Coverage Diff             @@
##           master     #778      +/-   ##
==========================================
- Coverage   88.21%   87.30%   -0.92%     
==========================================
  Files          12       12              
  Lines        4134     4097      -37     
==========================================
- Hits         3647     3577      -70     
- Misses        487      520      +33     

Impacted Files	Coverage Δ
R/fit.R	96.22% <100.00%> (+0.15%)	⬆️

... and 5 files with indirect coverage changes

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[topipa]

Thank you for implemeting this! Let me know if I can help with anything. The implementation looks good to me, although I'm not very familiar with cmdstanr.

[andrjohns]

Thank you for implemeting this! Let me know if I can help with anything. The implementation looks good to me, although I'm not very familiar with cmdstanr.

No worries! Feel free to let me know if there are any other additions/features that could be useful

[rok-cesnovar]

Looks good to me. Feel free to merge.

[jgabry]

Cool, thanks for implementing this!