json dump nested dicts and lists attrs

[OriolAbril]

Description

I have had some problems when saving to netcdf with the new extended attrs, seems related to pydata/xarray#2868. This uses json.dumps to convert the nested objects to strings that can be stored in netcdf and if necessary recovered by parsing the json again.

Checklist

• Follows official PR format
• Includes new or updated tests to cover the new feature. Should there be a writing test in test_pystan?
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[codecov]

Codecov Report

Merging #1417 into master will decrease coverage by 0.01%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #1417      +/-   ##
==========================================
- Coverage   91.57%   91.56%   -0.02%     
==========================================
  Files         105      105              
  Lines       11090    11097       +7     
==========================================
+ Hits        10156    10161       +5     
- Misses        934      936       +2     

Impacted Files	Coverage Δ
arviz/data/io_pystan.py	96.00% <71.42%> (-0.34%)	⬇️

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