allow from_pymc3 args to override model values

[OriolAbril]

Description

Also related (and blocker) of pymc-devs/pymc#3963

Checklist

• Follows official PR format
• New features are properly documented (with an example if appropriate)?
  • will be in radon notebook
• 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 #1249 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1249   +/-   ##
=======================================
  Coverage   93.11%   93.11%           
=======================================
  Files          98       98           
  Lines        9714     9715    +1     
=======================================
+ Hits         9045     9046    +1     
  Misses        669      669           

Impacted Files	Coverage Δ
arviz/data/io_pymc3.py	92.36% <100.00%> (+0.02%)	⬆️
arviz/plots/pairplot.py	92.77% <0.00%> (ø)

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[OriolAbril]

ready to merge :)