Adding Inline example for psislw and r2_score.

[smit-s]

Description

psislw and r2_score functions in stats.py did not have any inline examples. This PR adds inline example to these methods and addresses issue #1057.

Checklist

• Follows official PR format
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[codecov]

Codecov Report

Merging #1129 into master will increase coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1129      +/-   ##
==========================================
+ Coverage   92.67%   92.81%   +0.13%     
==========================================
  Files          93       93              
  Lines        9097     9181      +84     
==========================================
+ Hits         8431     8521      +90     
+ Misses        666      660       -6     

Impacted Files	Coverage Δ
arviz/stats/stats.py	96.32% <ø> (ø)
arviz/plots/traceplot.py	97.33% <0.00%> (-1.26%)	⬇️
arviz/data/inference_data.py	81.88% <0.00%> (-1.26%)	⬇️
arviz/utils.py	90.00% <0.00%> (-0.96%)	⬇️
arviz/plots/backends/bokeh/traceplot.py	95.97% <0.00%> (-0.43%)	⬇️
arviz/data/io_pystan.py	98.67% <0.00%> (-0.40%)	⬇️
arviz/rcparams.py	93.10% <0.00%> (ø)
arviz/plots/backends/__init__.py	29.11% <0.00%> (ø)
arviz/plots/backends/matplotlib/traceplot.py	99.15% <0.00%> (+0.06%)	⬆️
arviz/plots/backends/bokeh/rankplot.py	93.33% <0.00%> (+1.66%)	⬆️
... and 2 more

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[OriolAbril]

Get Pareto smoothed importance sampling (PSIS) log weights.

We should also add a reference, i.e. https://arxiv.org/abs/1507.02646 (see ess function for an example on the References section)

[ahartikainen]

Looks good, after addressing the reference issue, this is ready.