Add Inference data example with circular variables

[agustinaarroyuelo]

Description

This PR adds a new inference data example. It contains circular variables, which will be useful for testing the new features developed in my GSoC 2020 project.

Checklist

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

[codecov]

Codecov Report

Merging #1265 into master will increase coverage by 1.63%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1265      +/-   ##
==========================================
+ Coverage   91.58%   93.21%   +1.63%     
==========================================
  Files          98       98              
  Lines        9734     9734              
==========================================
+ Hits         8915     9074     +159     
+ Misses        819      660     -159     

Impacted Files	Coverage Δ
arviz/data/datasets.py	98.38% <ø> (ø)
arviz/data/io_emcee.py	98.33% <0.00%> (+2.50%)	⬆️
arviz/data/io_pystan.py	98.67% <0.00%> (+34.43%)	⬆️

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[aloctavodia]

maybe use example or dataset instead of object

[aloctavodia]

Thanks @agustinaarroyuelo