Issue 3549 psd diffusivity bug

[rtimms]

Description

Fixes a bug when using stoichiometry-dependent diffusivity with the DFN model with a particle size distribution

Fixes #3549

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 -m pytest (or $ nox -s tests)
• The documentation builds: $ python -m pytest --doctest-plus src (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ 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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.24%. Comparing base (a7253b8) to head (2651047).
Report is 135 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4726      +/-   ##
===========================================
+ Coverage    99.22%   99.24%   +0.01%     
===========================================
  Files          303      303              
  Lines        23070    23242     +172     
===========================================
+ Hits         22891    23066     +175     
+ Misses         179      176       -3     

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[brosaplanella]

Looks good, thanks!

[brosaplanella]

Why does the else mean it is evaluated at the edges?

[kratman]

@rtimms Hopefully the tests pass now