Allow differential surface form with MPM

[rtimms]

Description

Allow the option "surface form": "differential" in MPM

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

[valentinsulzer]

Can you add a unit and integration test for MPM with differential surface form

[codecov]

Codecov Report

Base: 99.73% // Head: 99.73% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (2c49aa3) compared to base (22e9d5a).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2533      +/-   ##
===========================================
- Coverage    99.73%   99.73%   -0.01%     
===========================================
  Files          270      270              
  Lines        19373    19365       -8     
===========================================
- Hits         19321    19313       -8     
  Misses          52       52              

Impacted Files	Coverage Δ
...bamm/models/full_battery_models/lithium_ion/mpm.py	100.00% <100.00%> (ø)

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[valentinsulzer]

Looks good, thanks!