Allow user to choose plotting library

[SarahRo]

Description

This PR adds the following features:

• All plotting functions can now be used with either plotly or matplotlib.pyplot as the plotting backend.
• The backend can be chosen by
  • calling `pybop.plot.use_backend('plotly')'
  • passing the backend as a keyword argument backend = ... to the plotting function.
  • creating a figure with either plotting library and passing it to the plotting function in the 'figures' keyword argument.
• Figures and axes can be passed as keyword arguments to plotting functions to allow full user control over figure layout and positioning within subplots.

Fixes #855

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #).

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: $ pre-commit run or $ nox -s pre-commit (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: nox -s tests
• The documentation builds: nox -s doctest
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.93%. Comparing base (3171a07) to head (0a5be84).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #928      +/-   ##
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+ Coverage    92.13%   92.93%   +0.79%     
===========================================
  Files           74       79       +5     
  Lines         5799     6367     +568     
===========================================
+ Hits          5343     5917     +574     
+ Misses         456      450       -6     

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[NicolaCourtier]

This is excellent, thanks @SarahRo !

Some minor comments and questions:

• do you think we should deprecate StandardPlot and StandardSubplot in favour of the standardised backend functionality, since they only seem to be used once each?
• there seems to be an issue with the Voronoi plot in optimising_with_scipy_minimize.py although it works elsewhere, please could you check if this is due to changes made in this PR?

[SarahRo]

This is excellent, thanks @SarahRo !

Some minor comments and questions:

* do you think we should deprecate `StandardPlot` and `StandardSubplot` in favour of the standardised backend functionality, since they only seem to be used once each?

* there seems to be an issue with the Voronoi plot in `optimising_with_scipy_minimize.py` although it works elsewhere, please could you check if this is due to changes made in this PR?

I'm looking into this. I simplified the voronoi plot when replicating it in matplotlib so that is likely what is causing the issue. It must be a bit of an edge case since it's working for most examples.

I will also look into the changes required for deprecating StandardPlot and StandardSubplot.

[NicolaCourtier]

Looks great, thanks @SarahRo !