Issue 984 butler volmer resistance

[valentinsulzer]

Description

Fix some bugs in the SEI submodels and add the SEI film resistance.

Note: we may want to think about the nondimensionalisation for the SEI a bit more

Fixes #984

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 #994 into develop will increase coverage by 0.01%.
The diff coverage is 98.50%.

@@             Coverage Diff             @@
##           develop     #994      +/-   ##
===========================================
+ Coverage    97.56%   97.58%   +0.01%     
===========================================
  Files          232      234       +2     
  Lines        12119    12251     +132     
===========================================
+ Hits         11824    11955     +131     
- Misses         295      296       +1     

Impacted Files	Coverage Δ
...bamm/models/full_battery_models/lithium_ion/dfn.py	100.00% <ø> (ø)
pybamm/parameters/parameter_sets.py	100.00% <ø> (ø)
pybamm/parameters/parameter_values.py	99.56% <80.00%> (-0.08%)	⬇️
pybamm/models/submodels/interface/sei/base_sei.py	95.91% <83.33%> (ø)
...ybamm/models/submodels/interface/base_interface.py	99.59% <95.00%> (-0.41%)	⬇️
...m/models/full_battery_models/base_battery_model.py	99.62% <100.00%> (+0.02%)	⬆️
...l_battery_models/lead_acid/base_lead_acid_model.py	100.00% <100.00%> (ø)
...ybamm/models/full_battery_models/lead_acid/full.py	100.00% <100.00%> (ø)
...dels/full_battery_models/lead_acid/higher_order.py	98.07% <100.00%> (+0.01%)	⬆️
...ybamm/models/full_battery_models/lead_acid/loqs.py	100.00% <100.00%> (ø)
... and 31 more

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[brosaplanella]

Thanks Tino! It looks great :D