fix bug in composite surface form model

[rtimms]

Description

Fixes a bug where a factor of electrode surface area to volume ratio is missing in the rhs of the CompositeDifferential conductivity model.

In particular, this fixes the missing semi-circle when simulating EIS with SPMe. This got missed in #4139

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

[rtimms]

@kratman @Saransh-cpp can we squeeze this bug fix into 24.5 or if it's simpler do a patch?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.45%. Comparing base (ba39170) to head (a5f7bb0).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4293   +/-   ##
========================================
  Coverage    99.45%   99.45%           
========================================
  Files          288      288           
  Lines        22091    22092    +1     
========================================
+ Hits         21971    21972    +1     
  Misses         120      120           

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[kratman]

I will cherry-pick this into the release