Better Loss Plot

[jerrymhuang]

Fixed and refined bf.diagnostics.plots.loss:

• Replace the moving average with an exponential moving average and make it functional and by default;
• Provide the user with a new set of customization, including moving average window (span), color maps, markers, and grid;
• Make the validation loss a viridis gradient line.

This version of loss plot also propagates to the workflow object.

[codecov]

Codecov Report

Attention: Patch coverage is 40.42553% with 28 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
bayesflow/utils/plot_utils.py	21.21%	26 Missing ⚠️
bayesflow/diagnostics/plots/loss.py	85.71%	2 Missing ⚠️

Files with missing lines	Coverage Δ
bayesflow/workflows/basic_workflow.py	61.40% <ø> (ø)
bayesflow/diagnostics/plots/loss.py	92.68% <85.71%> (+20.46%)	⬆️
bayesflow/utils/plot_utils.py	68.42% <21.21%> (-19.24%)	⬇️

... and 15 files with indirect coverage changes

[LarsKue]

Looks great! I will add the changes that I proposed myself.

[LarsKue]

Although it is very pretty, I am removing the gradient line for now, until we have resolved how we can reliably add it to the legend. I also simplified the rest of the code and fixed an issue with the smoothing factor being applied the wrong way around.