[WIP] Add plot_trace_dens

[aloctavodia]

This is my attempt to reproduce the legacy plot_trace.

we have ecdf too!

And also a lot of mistakes.

• I added the function scale_fig_size (needs more works). Is this the approach we are going to use for the new ArviZ?
• I use matplotlib keywords to get/set default values, which will fail for bokeh or other backends.
• compact=true is not doing the right thing (I guess school is stacked ), see below:

• I added some logic to set the size of ticks, remove ticks and add x/y labels I am not sure I am doing the right thing, in particular for setting the size of ticks and remove ticks.

• I did not add any code for bokeh...

• Docstring and even variable names may need to be cleaned...

• Should we call this plot_trace_dist instead of plot_dens?

• To reduce redundancy and make the plots more compact, we may want to have a title per row with the variable names and spanning both columns. Not sure how easy is that

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📚 Documentation preview 📚: https://arviz-plots--27.org.readthedocs.build/en/27/

[codecov-commenter]

Codecov Report

Attention: 102 lines in your changes are missing coverage. Please review.

Comparison is base (e1a36c5) 76.22% compared to head (eff66bd) 65.65%.

Files	Patch %	Lines
src/arviz_plots/plots/tracedistplot.py	10.29%	61 Missing :warning:
src/arviz_plots/plots/utils.py	8.69%	21 Missing :warning:
src/arviz_plots/backend/matplotlib/__init__.py	25.00%	12 Missing :warning:
src/arviz_plots/visuals/__init__.py	33.33%	8 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #27 +/- ## =========================================== - Coverage 76.22% 65.65% -10.57% =========================================== Files 10 11 +1 Lines 572 693 +121 =========================================== + Hits 436 455 +19 - Misses 136 238 +102 ```

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[OriolAbril]

Should we call this plot_trace_dist instead of plot_dens

me gusta más con dist, si

[aloctavodia]

Merging this. We can keep working on separate PRs, I will open issues.