Added new parameter z_sei to allow the user to set SEI valancy

[DrSOKane]

Description

A new parameter "Ratio of lithium moles to SEI moles" (short name z_sei) has been added. This variable was previously hardcoded to 1 for all SEI growth options except "ec reaction limited". As pointed out in issue #2065, z_sei=1 is incorrect for the two most common SEI compoints - LEDC and Li2CO3, both of which require z=2.

The SEI parameter sets have been updated accordingly. The new parameter z_sei is set to 2 by default for all parameter sets except OKane2022, which used the old code for which it was set to 1.

Fixes #2065

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 #2222 (86c1608) into develop (2fd114e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 86c1608 differs from pull request most recent head 7e529b1. Consider uploading reports for the commit 7e529b1 to get more accurate results

@@           Coverage Diff            @@
##           develop    #2222   +/-   ##
========================================
  Coverage    99.36%   99.36%           
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  Files          355      355           
  Lines        19499    19499           
========================================
+ Hits         19375    19376    +1     
+ Misses         124      123    -1     

Impacted Files	Coverage Δ
pybamm/models/submodels/interface/sei/base_sei.py	100.00% <100.00%> (ø)
...ybamm/models/submodels/interface/sei/sei_growth.py	100.00% <100.00%> (ø)
pybamm/parameters/lithium_ion_parameters.py	100.00% <100.00%> (ø)
pybamm/expression_tree/symbol.py	98.79% <0.00%> (+0.24%)	⬆️

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[valentinsulzer]

I don't see beta_sei and beta_plating being used anywhere

[DrSOKane]

I don't see beta_sei and beta_plating being used anywhere

I remember beta_sei being a legacy parameter from when the porosity submodel was an ODE, because I created beta_plating using the same logic. The porosity submodel seems to be ever-changing so is it worth keeping these?

[valentinsulzer]

Let's remove them, it's easy to add back in if needed, and confusing otherwise