plot_ppc add observed argument to (un)plot observed data

[aloctavodia]

• 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

I also made a few other changes in bokeh, specially to use the matplotlib color and to make the scatter plot a little fit faster.

[codecov]

Codecov Report

Merging #1422 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1422   +/-   ##
=======================================
  Coverage   91.56%   91.57%           
=======================================
  Files         105      105           
  Lines       11098    11107    +9     
=======================================
+ Hits        10162    10171    +9     
  Misses        936      936           

Impacted Files	Coverage Δ
arviz/plots/backends/bokeh/ppcplot.py	97.69% <100.00%> (+0.11%)	⬆️
arviz/plots/backends/matplotlib/ppcplot.py	98.10% <100.00%> (+0.02%)	⬆️
arviz/plots/ppcplot.py	93.44% <100.00%> (ø)

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[OriolAbril]

Suggested change

	mean: bool
	Whether or not to plot the observed data. Defaults to True
	observed: bool, default True
	Whether or not to plot the observed data.