Update grouped models

[NicolaCourtier]

Description

The main additions are:

• voltage components for each grouped model to enable PyBaMM's plot_voltage_components
• asymmetric and multiphase Butler-Volmer kinetics

Smaller changes:

• label SPDiffusion and WeppnerHuggins models as li_half_cell models with a "positive" domain
• remove the adjustment of the OCV that forces it to blow up at the stoichiometry limits
• use the "U__c_s" tolerance for the stoichiometric event tolerance instead
• get the reaction rate parameter values from PyBaMM exchange current densities
• use relative transport efficiencies instead of electrolyte diffusion times for each layer

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #).

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: $ pre-commit run or $ nox -s pre-commit (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: nox -s tests
• The documentation builds: nox -s doctest
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

❌ Patch coverage is 94.36620% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.90%. Comparing base (3171a07) to head (9f84322).
⚠️ Report is 11 commits behind head on develop.

Files with missing lines	Patch %	Lines
pybop/models/alternative_functions.py	80.00%	5 Missing ⚠️
pybop/models/lithium_ion/grouped_spm.py	91.30%	4 Missing ⚠️
pybop/models/lithium_ion/grouped_spme.py	92.98%	4 Missing ⚠️
pybop/applications/gitt_methods.py	0.00%	2 Missing ⚠️
pybop/models/li_half_cell/base_model.py	96.87%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #974      +/-   ##
===========================================
+ Coverage    92.13%   92.90%   +0.76%     
===========================================
  Files           74       84      +10     
  Lines         5799     6777     +978     
===========================================
+ Hits          5343     6296     +953     
- Misses         456      481      +25     

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