fix io_pymc3 bug when model not passed explicitly

[OriolAbril]

Description

if model is not passed explicitly, it is retrieved from the context and stored in self.model, however, currently we check coords/dims in model instead.

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 #1240 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1240   +/-   ##
=======================================
  Coverage   93.16%   93.16%           
=======================================
  Files          98       98           
  Lines        9714     9714           
=======================================
  Hits         9050     9050           
  Misses        664      664           

Impacted Files	Coverage Δ
arviz/data/io_pymc3.py	93.79% <100.00%> (ø)

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[AlexAndorra]

LGTM, thanks Oriol 👌

[OriolAbril]

ready to merge!