add plot_trace

[aloctavodia]

Closes #13

This follows the decision that plot_trace will be a plot without the densities (kde or histograms).

I started with this example because it seems simple enough to get familiar with the code. Please let me know what I am doing wrong.

Also not sure what should I do to add divergences, given that divergences are defined in a separate group.

This is how the default plots look like for the centered_eight dataset

and this is with some customization

plot_trace(post, var_names=["mu", "tau"],
sample_dims=["draw"],
pc_kwargs={"plot_grid_kws": {"figsize": (12, 7), "sharey":"row"}, "col_wrap":4})

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📚 Documentation preview 📚: https://arviz-plots--1.org.readthedocs.build/en/1/

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[OriolAbril]

Also not sure what should I do to add divergences, given that divergences are defined in a separate group.

There is no clear way to go about that yet. I think the clearest way to go about it is allowing to define different data when looping/facetting than when plotting

[aloctavodia]

Is this ok to merge? Or what else should I do?