Explicit index support in PlotCollection.map

[OriolAbril]

See the compose your own plot page for proof of concept.

Closes #237 and should unblock #149 (yet not fix it though as we'll still need to handle the kde/ecdf... computation through groupby for that to work completely).

Also closes #257

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📚 Documentation preview 📚: https://arviz-plots--241.org.readthedocs.build/en/241/

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.12821% with 97 lines in your changes missing coverage. Please review.

Project coverage is 72.44%. Comparing base (544e7f1) to head (5d392a4).

Files with missing lines	Patch %	Lines
src/arviz_plots/plot_collection.py	83.40%	39 Missing ⚠️
src/arviz_plots/backend/none/__init__.py	29.16%	17 Missing ⚠️
src/arviz_plots/backend/plotly/__init__.py	44.82%	16 Missing ⚠️
src/arviz_plots/backend/bokeh/__init__.py	52.17%	11 Missing ⚠️
src/arviz_plots/visuals/__init__.py	83.33%	8 Missing ⚠️
src/arviz_plots/plots/combine.py	33.33%	4 Missing ⚠️
src/arviz_plots/backend/matplotlib/__init__.py	80.00%	1 Missing ⚠️
src/arviz_plots/plot_matrix.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #241      +/-   ##
==========================================
+ Coverage   71.02%   72.44%   +1.41%     
==========================================
  Files          42       42              
  Lines        4860     4921      +61     
==========================================
+ Hits         3452     3565     +113     
+ Misses       1408     1356      -52     

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[OriolAbril]

I ended up extending the behaviour to coordinate values and we internally call set_xindex and use dim_to_idx if needed. That being said, I haven't checked how expensive it is but it is definitely better to call set_xindex once, not every time .map is called or a subset of a kwarg requested so we'll need to document that somehow.

Faceting and aes now work from coordinate values. https://arviz-plots--241.org.readthedocs.build/en/241/tutorials/compose_own_plot.html has a demonstration of the behaviour (or will as the rtd build is still in progress as I write this). I have also solved a couple smaller issues in the process, mainly the fn(da, target, backend, ...) to fn(da, target, ...) and renamed all instances of "chart" to "figure".

Needs more testing and documentation (existing ones have been fixed/updated as necessary), but if there are incoming PRs it might be better to merge and add them in a follow up PR to avoid git conflicts.

[OriolAbril]

Legends also need to be fixed to adapt to the new aesthetics logic

[OriolAbril]

Legends fixed, and if I say so myself the new structure allows for much better legends without as much complexity