bugfix(double-layer cap scaling): rescaling overpotential RHS by 1/a

[bardsleypt]

Description

Rescaling RHS of overpotential differential equation by 1/a to correct for proper non-dimensionalization for non-constant specific-surface areas (i.e., non-constant active material volume fractions and/or particle radii).

Changes include:

• Updating full_surface_form_conductivity RHS to rescale RHS of
  negative-electrode overpotential DE by a factor of 1/a, which
  remains present upon non-dimensionalization
• 1/a scaling only affects DFN models with 'surface form' =
  'differential' and any of the following:
  • non-constant particle radii across electrodes
  • non-constant active material volume fractions across electrodes
  • loss of active material enabled
  • other forms of non-constant specific surface areas 'a'

See Discussion #2142 for further details

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: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --quick.

Further checks:

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

[bardsleypt]

I ran the tests, but a lot of the casadi interface tests seemt to fail - likely because of my install. I have had a hard time getting the dev install to run successfully for an M1 mac - pip doesn't want to install scikits.odes so manual installs and tox installs all fail. Any advice on this is appreciated, otherwise maybe someone can check the tests for me on a proper dev-installed machine?

Figured for this PR its a small enough change that it shouldn't be causing significant issues, at least not that can't be obviously tracked (failing surface-form unit test for example).

[valentinsulzer]

The CI will catch any failing tests, I would be very very surprised if these changes caused tests to fail

[codecov]

Codecov Report

Merging #2175 (e92487d) into develop (d4c5880) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #2175   +/-   ##
========================================
  Coverage    99.35%   99.35%           
========================================
  Files          356      356           
  Lines        19357    19357           
========================================
  Hits         19233    19233           
  Misses         124      124           

Impacted Files	Coverage Δ
...e_potential_form/full_surface_form_conductivity.py	100.00% <100.00%> (ø)

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[valentinsulzer]

@all-contributors add @bardsleypt for bugs, code

[allcontributors]

@tinosulzer

I've put up a pull request to add @bardsleypt! 🎉