Circular traceplot for matplotlib

[agustinaarroyuelo]

Description

In this PR I am adding argument circ_var_names to plot_trace in order to obtain a plot like this:

Checklist

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• New features are properly documented (with an example if appropriate)?
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[aloctavodia]

Could you also add one example to the example gallery?

[ahartikainen]

LGTM

[codecov]

Codecov Report

Merging #1336 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1336      +/-   ##
==========================================
+ Coverage   91.77%   91.79%   +0.02%     
==========================================
  Files         101      101              
  Lines       10622    10634      +12     
==========================================
+ Hits         9748     9762      +14     
+ Misses        874      872       -2     

Impacted Files	Coverage Δ
arviz/plots/backends/bokeh/traceplot.py	94.44% <ø> (ø)
arviz/plots/traceplot.py	95.12% <ø> (ø)
arviz/plots/backends/matplotlib/traceplot.py	99.36% <100.00%> (+0.05%)	⬆️
arviz/plots/backends/bokeh/forestplot.py	92.85% <0.00%> (+0.32%)	⬆️
arviz/plots/backends/matplotlib/forestplot.py	97.92% <0.00%> (+0.34%)	⬆️

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