introduce VariableArray class

[paul-buerkner]

This PR further improves the diagnostics module:

• Introduces a basic VariableArray class to carry around variable_keys and variable_names with an array containing posterior draws. This class is only meant for internal use for now and we can see if extending it in the future could be sensible.

• Further improves posterior pairs plots to enable plotting the prior again.

For example, using the linear regression notebook, we can now do something like:

bf.diagnostics.plots.pairs_posterior(
    estimates=post_draws, 
    targets=val_sims,
    priors=sim_draws,
    dataset_id=0,
    variable_names=par_names,
)
plt.show()

to show both posterior, prior, and the true parameter values. The previous implementation didn't quite work in terms of correctly showing the prior.

There is one issue which I couldn't quite resolve and I would like to get your input @stefanradev93 and @jerrymhuang: Currently, the posterior is always plotted behind the prior on the diagonal. I tried to change that via hue_order for example but it either breaks the off diagonal plots or mislabels prior and posterior. It kind of looks like a bug in seaborn to me as I cannot imagine this to be intended. Any ideas would be highly appreciated.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 6.66667% with 70 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
bayesflow/diagnostics/plots/pairs_samples.py	0.00%	30 Missing ⚠️
bayesflow/utils/dict_utils.py	16.00%	21 Missing ⚠️
bayesflow/diagnostics/plots/pairs_posterior.py	11.11%	8 Missing ⚠️
bayesflow/utils/plot_utils.py	0.00%	5 Missing ⚠️
...sflow/diagnostics/metrics/posterior_contraction.py	0.00%	2 Missing ⚠️
...low/diagnostics/metrics/root_mean_squared_error.py	0.00%	2 Missing ⚠️
bayesflow/diagnostics/metrics/calibration_error.py	0.00%	1 Missing ⚠️
bayesflow/diagnostics/plots/calibration_ecdf.py	0.00%	1 Missing ⚠️

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Files with missing lines	Coverage Δ
bayesflow/diagnostics/metrics/calibration_error.py	21.05% <0.00%> (-1.17%)	⬇️
bayesflow/diagnostics/plots/calibration_ecdf.py	14.89% <0.00%> (ø)
...sflow/diagnostics/metrics/posterior_contraction.py	36.36% <0.00%> (-3.64%)	⬇️
...low/diagnostics/metrics/root_mean_squared_error.py	30.76% <0.00%> (-2.57%)	⬇️
bayesflow/utils/plot_utils.py	19.75% <0.00%> (-1.03%)	⬇️
bayesflow/diagnostics/plots/pairs_posterior.py	37.50% <11.11%> (+5.50%)	⬆️
bayesflow/utils/dict_utils.py	21.25% <16.00%> (-1.39%)	⬇️
bayesflow/diagnostics/plots/pairs_samples.py	16.32% <0.00%> (-5.90%)	⬇️

[paul-buerkner]

I think I now fixed also the histogram issue to the degree that seaborn allows. I think this PR is ready to merge once @stefanradev93 approves.