Fix incorrect format of inv_metric when only 1 parameter in model

[venpopov]

Submission Checklist

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

Summary

Fixes #934. Now it checks in inv_matrix() function whether the input is a scalar for each chain, and if so, makes it an array with dimension 1. It also corrects the behavior of diag() when matrix=TRUE, which previously produced an empty matrix with a scalar inv_metric.

Copyright and Licensing

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Vencislav Popov

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]

This looks good, thanks! I think the windows-latest (devel) failure is unrelated to this.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.33%. Comparing base (82f9d9a) to head (41dbaf8).

❗ Current head 41dbaf8 differs from pull request most recent head c69ef19. Consider uploading reports for the commit c69ef19 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #935      +/-   ##
==========================================
+ Coverage   88.32%   88.33%   +0.01%     
==========================================
  Files          12       12              
  Lines        4549     4553       +4     
==========================================
+ Hits         4018     4022       +4     
  Misses        531      531              

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