Add the GroupedDFN with flux boundary conditions

[NicolaCourtier]

Description

Add the GroupedDFN and update the other lithium_ion models to use conservative "Flux" boundary conditions. Also, enable the electrolyte diffusivity to be a function.

⚠️ Temporarily using my fork of PyBaMM until the resolution of PyBaMM issue 5700.

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 98.07692% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.10%. Comparing base (3171a07) to head (cc19ffd).
⚠️ Report is 13 commits behind head on develop.

Files with missing lines	Patch %	Lines
pybop/models/lithium_ion/grouped_dfn.py	97.79%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #977      +/-   ##
===========================================
+ Coverage    92.13%   93.10%   +0.96%     
===========================================
  Files           74       85      +11     
  Lines         5799     7017    +1218     
===========================================
+ Hits          5343     6533    +1190     
- Misses         456      484      +28     

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