allow overlaid pairplots

[OriolAbril]

Description

Fix issue #1130.

Checklist

• Follows official PR format
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[codecov]

Codecov Report

Merging #1135 into master will increase coverage by 0.21%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master    #1135      +/-   ##
==========================================
+ Coverage   92.67%   92.89%   +0.21%     
==========================================
  Files          93       93              
  Lines        9097     9102       +5     
==========================================
+ Hits         8431     8455      +24     
+ Misses        666      647      -19     

Impacted Files	Coverage Δ
arviz/plots/backends/matplotlib/pairplot.py	85.61% <90.00%> (+14.71%)	⬆️
arviz/data/converters.py	68.57% <100.00%> (ø)

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[ahartikainen]

LGTM

[ahartikainen]

I noticed changelog needs updating, but let's do that in another PR then