use posterior::autocovariance if available

[jgabry]

advances #157

[codecov-commenter]

Codecov Report

Merging #219 (27398c6) into master (58fe5be) will decrease coverage by 0.65%.
The diff coverage is 66.66%.

❗ Current head 27398c6 differs from pull request most recent head f32418c. Consider uploading reports for the commit f32418c to get more accurate results

@@            Coverage Diff             @@
##           master     #219      +/-   ##
==========================================
- Coverage   93.65%   93.01%   -0.65%     
==========================================
  Files          30       30              
  Lines        2775     2777       +2     
==========================================
- Hits         2599     2583      -16     
- Misses        176      194      +18     

Impacted Files	Coverage Δ
R/effective_sample_sizes.R	70.54% <66.66%> (-13.71%)	⬇️

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