fix bug in plot_loo_pit with use_hdi

[OriolAbril]

Description

Checklist

• Follows official PR format
• Changes are listed in changelog

[aloctavodia]

My original intention was to create a separated PR, but maybe we can go with this.

This makes two important changes. compute the kde over the data range (not over 0, 1). This help to spots problems, as the KDE is "not adding data". Compare our example in the gallery, with the new version.

.

This also introduce a theoretical HDI, instead of estimating the HDI from the samples.

maybe we should enforce the zero y-bottom-limit.

[codecov]

Codecov Report

Merging #1214 into master will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1214      +/-   ##
==========================================
- Coverage   93.14%   93.07%   -0.07%     
==========================================
  Files          94       94              
  Lines        9497     9503       +6     
==========================================
- Hits         8846     8845       -1     
- Misses        651      658       +7     

Impacted Files	Coverage Δ
arviz/plots/backends/bokeh/loopitplot.py	89.18% <100.00%> (+0.30%)	⬆️
arviz/plots/backends/matplotlib/loopitplot.py	97.22% <100.00%> (+0.44%)	⬆️
arviz/plots/loopitplot.py	98.87% <100.00%> (ø)
arviz/plots/hdiplot.py	83.92% <0.00%> (-3.58%)	⬇️
arviz/utils.py	89.33% <0.00%> (-1.84%)	⬇️

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[aloctavodia]

Sorry for the mess I am closing this in favor of #1215