Fix labels and use circular KDE in plot_trace

[agustinaarroyuelo]

Description

I noticed that the KDE plot in plot_trace was not using the circular KDE function (#1284). I also fixed the labels so that both columns share the same units.

Degrees:

Radians:

Checklist

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[codecov]

Codecov Report

Merging #1428 into master will increase coverage by 0.35%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1428      +/-   ##
==========================================
+ Coverage   91.58%   91.94%   +0.35%     
==========================================
  Files         105      105              
  Lines       11125    11132       +7     
==========================================
+ Hits        10189    10235      +46     
+ Misses        936      897      -39     

Impacted Files	Coverage Δ
arviz/plots/backends/matplotlib/kdeplot.py	98.75% <ø> (ø)
arviz/plots/backends/matplotlib/traceplot.py	98.80% <100.00%> (+0.05%)	⬆️
arviz/plots/kdeplot.py	100.00% <0.00%> (+3.12%)	⬆️
arviz/stats/density_utils.py	75.07% <0.00%> (+10.76%)	⬆️

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