Keep model_params to avoid breaking brms

[jgabry]

Submission Checklist

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

Summary

closes #1151

Keep model_params in metadata for now due to usage in brms. See #1151 for details. Thanks to @anirban-mu for reporting the issue.

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):
Jonah Gabry

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.97%. Comparing base (a8adbb1) to head (c27bc84).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1152   +/-   ##
=======================================
  Coverage   87.97%   87.97%           
=======================================
  Files          14       14           
  Lines        5937     5938    +1     
=======================================
+ Hits         5223     5224    +1     
  Misses        714      714           

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