Print unequal sample sizes in loo_compare

[VisruthSK]

Fixes #296.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.78%. Comparing base (b4c12c3) to head (bab8827).
⚠️ Report is 37 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #307      +/-   ##
==========================================
+ Coverage   92.76%   92.78%   +0.01%     
==========================================
  Files          31       31              
  Lines        2986     2992       +6     
==========================================
+ Hits         2770     2776       +6     
  Misses        216      216              

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[jgabry]

Thanks! This looks good at first glance but I haven't had a chance to try it out yet. Busy day for me today but will try it out as soon as I can.

[jgabry]

I just added one more test to also include the scenario where the user supplies custom model names (by providing the models as a named list).

[jgabry]

I think this is ready to merge once the checks pass (I just pushed one commit so rerunning them)