Implementation of matplotlib backend for criterion_plot()

[r3kste]

Summary of Changes

1. Added backend parameter to criterion_plot() which accepts "plotly" or "matplotlib"
2. criterion_plot() now returns the figure object corresponding to the chosen backend.
3. Added backends.py which serves as a registry for all backends

To-Do

It is very much still a work in progress.

• Discuss on whether the dispatch mechanism can be improved.
• Add examples in documentation
• Add suitable tests

Plot

Plotly	Matplotlib

[codecov]

Codecov Report

Attention: Patch coverage is 84.31373% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/optimagic/visualization/backends.py	78.87%	15 Missing ⚠️
src/optimagic/visualization/plotting_utilities.py	93.33%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/optimagic/config.py	70.27% <100.00%> (+0.82%)	⬆️
src/optimagic/visualization/history_plots.py	92.51% <100.00%> (+1.66%)	⬆️
src/optimagic/visualization/plotting_utilities.py	94.11% <93.33%> (-0.15%)	⬇️
src/optimagic/visualization/backends.py	78.87% <78.87%> (ø)

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