feature/esoh model update

[anoushka2000]

Description

Fixes #1856 and #1842 (issue)

Makes electrode state of health model more robust by calculating initial conditions based on model parameters and solving model in two stages (separating solution for stoichiometry x_100 and capacity C).

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 #2109 (6d69982) into develop (6877eac) will decrease coverage by 0.07%.
The diff coverage is 89.24%.

@@             Coverage Diff             @@
##           develop    #2109      +/-   ##
===========================================
- Coverage    99.38%   99.30%   -0.08%     
===========================================
  Files          356      356              
  Lines        19297    19409     +112     
===========================================
+ Hits         19178    19274      +96     
- Misses         119      135      +16     

Impacted Files	Coverage Δ
...s/full_battery_models/lithium_ion/electrode_soh.py	90.74% <87.34%> (-9.26%)	⬇️
...models/full_battery_models/lithium_ion/__init__.py	100.00% <100.00%> (ø)
pybamm/simulation.py	98.82% <100.00%> (+<0.01%)	⬆️
pybamm/solvers/solution.py	100.00% <100.00%> (ø)
pybamm/models/base_model.py	98.37% <0.00%> (-0.89%)	⬇️
...m/models/full_battery_models/base_battery_model.py	99.75% <0.00%> (-0.02%)	⬇️
...ybamm/models/full_battery_models/lead_acid/full.py	100.00% <0.00%> (ø)
...ybamm/models/full_battery_models/lead_acid/loqs.py	100.00% <0.00%> (ø)
...bamm/models/full_battery_models/lithium_ion/dfn.py	100.00% <0.00%> (ø)
... and 8 more

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