Surface form half cell

[valentinsulzer]

Description

Fixes half-cell models so that they can be run with "surface form"

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

[codecov]

Codecov Report

Merging #1913 (cf16770) into develop (dc4ff98) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1913      +/-   ##
===========================================
+ Coverage    99.31%   99.32%   +0.01%     
===========================================
  Files          346      346              
  Lines        19105    19124      +19     
===========================================
+ Hits         18974    18995      +21     
+ Misses         131      129       -2     

Impacted Files	Coverage Δ
pybamm/solvers/casadi_solver.py	98.23% <ø> (ø)
...m/models/full_battery_models/base_battery_model.py	99.75% <100.00%> (ø)
...ybamm/models/full_battery_models/lead_acid/full.py	100.00% <100.00%> (ø)
...ttery_models/lithium_ion/base_lithium_ion_model.py	100.00% <100.00%> (ø)
...bamm/models/full_battery_models/lithium_ion/dfn.py	100.00% <100.00%> (ø)
...bamm/models/full_battery_models/lithium_ion/spm.py	100.00% <100.00%> (ø)
pybamm/models/submodels/electrode/ohm/li_metal.py	100.00% <100.00%> (ø)
...models/submodels/electrode/ohm/surface_form_ohm.py	100.00% <100.00%> (ø)
...e_potential_form/full_surface_form_conductivity.py	100.00% <100.00%> (+1.48%)	⬆️
...nterface/interface_utilisation/base_utilisation.py	100.00% <100.00%> (ø)
... and 2 more

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[rtimms]

thanks @tinosulzer looks good!