Adds Voronoi cost landscape plot

[BradyPlanden]

Description

• Adds a Voronoi cost landscape plot (pybop.plot.surface)
• Refactors Optimiser.log for consistent object types
• Breaking change for plotting. pybop.plot_XXX methods become pybop.plot.XXX

Plots

Here's the voronoi for a particle swarm parameterisation workflow. The voronoi works well for the heuristic / global optimisation algorithms, as they tend to explore the landscape.

For gradient based optimisers, the landscape is less informative. I believe this is intentionally helpful to the user, as it actually conveys the lack of information we know about the search space.

This PR should probably have a follow up that improves the documentation on how to use this plot, as it's not very common.

Issue reference

Fixes #361

Review

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Type of change

• [X] New Feature: A non-breaking change that adds new functionality.
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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 99.48980% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.09%. Comparing base (c8b00e6) to head (1315108). Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
pybop/plot/voronoi.py	99.07%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #531 +/- ## ========================================= Coverage 99.08% 99.09% ========================================= Files 52 54 +2 Lines 3605 3752 +147 ========================================= + Hits 3572 3718 +146 - Misses 33 34 +1 ```

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[BradyPlanden]

Thanks @NicolaCourtier, this should be ready for another review after the tests pass.

Any input on the method name? I'm torn between something simple like plot_surface or more informative plot_voronoi2d. Perhaps, surface is more understandable for users.. For reference, Pints call their implementation surface. I'll also replace more of the plot2d calls in the examples with this once we are happy with the name, as it's faster and more representative of the cost landscape from the optimisation perspective.

[NicolaCourtier]

Thanks, I completely agree with updating the default 2d plot to this one for both speed and accuracy. I'd be happy with either plot_surface or plot_voronoi/2d, but then I think we should also update the name of plot2d to distinguish between the two, perhaps this could become plot_contours?

[BradyPlanden]

This now has a breaking change for the plotting methods as described in the changelog. Essentially, pybop.plot_XXX becomes pybop.plot.XXX where possible. This adds clearer separation for the plotting methods, and allows for better method naming. A few of the methods have a larger rename, such as, plot2d becoming plot.contour and plot_voronoi_2d is now plot.surface