fixes to matplotlib subplot handling

[OriolAbril]

Description

Minor changes to matplotlib subplot handling in plot_trace and plot_pair.

In plot_trace plot.max_subplots used to be checked on the number of rows in plot trace, this PR fixes it to check the actual number of subplots.

I also checked the behaviour in plot_pair to check that it takes into account the actual number of used subplots with or without marginals. I have also modified axes creation to avoid having extra blank spaces at the top and right of with marginals=False (see https://arviz-devs.github.io/arviz/examples/matplotlib/mpl_plot_pair.html for example)

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 #1205 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1205   +/-   ##
=======================================
  Coverage   93.10%   93.10%           
=======================================
  Files          94       94           
  Lines        9480     9480           
=======================================
  Hits         8826     8826           
  Misses        654      654           

Impacted Files	Coverage Δ
arviz/plots/backends/matplotlib/pairplot.py	90.64% <100.00%> (ø)
arviz/plots/traceplot.py	97.33% <100.00%> (ø)

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