Warn when neff ratios exceed 1

[utkarshpawade]

Fixes #494

Summary

• validate_neff_ratio() now issues an rlang::warn() when any values are greater than 1
• Updated existing tests and added a new expect_warning() assertion

[jgabry]

With Stan when ESS (n_eff) exceeds the posterior sample size it is actually often a favorable sign, not a bad one, so I don't actually think we need a warning here.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.66%. Comparing base (a30a706) to head (94c89e0).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #495   +/-   ##
=======================================
  Coverage   98.66%   98.66%           
=======================================
  Files          35       35           
  Lines        5857     5862    +5     
=======================================
+ Hits         5779     5784    +5     
  Misses         78       78           

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