add grid argument to manually specify the number of rows and colums

[aloctavodia]

Fix #827

• Follows official PR format
• New features are properly documented (with an example if appropriate)?
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[codecov]

Codecov Report

Merging #1459 (4a657a9) into master (d67c252) will decrease coverage by 0.04%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #1459      +/-   ##
==========================================
- Coverage   91.93%   91.89%   -0.05%     
==========================================
  Files         105      105              
  Lines       11228    11235       +7     
==========================================
+ Hits        10323    10324       +1     
- Misses        905      911       +6     

Impacted Files	Coverage Δ
arviz/plots/plot_utils.py	89.37% <63.15%> (-2.13%)	⬇️
arviz/plots/autocorrplot.py	100.00% <100.00%> (ø)
arviz/plots/bpvplot.py	79.24% <100.00%> (ø)
arviz/plots/densityplot.py	88.88% <100.00%> (ø)
arviz/plots/essplot.py	100.00% <100.00%> (ø)
arviz/plots/mcseplot.py	100.00% <100.00%> (ø)
arviz/plots/posteriorplot.py	87.50% <100.00%> (ø)
arviz/plots/ppcplot.py	93.44% <100.00%> (ø)
arviz/plots/rankplot.py	90.32% <100.00%> (ø)
arviz/plots/violinplot.py	83.33% <100.00%> (ø)

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