Graded electrode example

[brosaplanella]

Description

Not related to any issue (sorry!), but I was asked how to simulate graded electrodes and realised we didn't have an example, so adding one here.

I kept it as a quite basic example, but if people have suggestions on other cases to study let me know.

Fixes # (issue)

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• 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: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.41%. Comparing base (ee325c7) to head (b3aeaf1).
Report is 202 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4424   +/-   ##
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  Coverage    99.41%   99.41%           
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  Files          292      292           
  Lines        22212    22212           
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  Hits         22083    22083           
  Misses         129      129           

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[valentinsulzer]

Looks good, thanks!