Update set_initial_state

[NicolaCourtier]

Description

Update the set_initial_state functions for models that can take a initial voltage and determine the initial state of charge or stoichiometry. Also, move the Interpolant and InverseOCV classes into a new model utils.py.

Type of change

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

Important checks:

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• 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)
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• 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 99.26471% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.31%. Comparing base (4628a65) to head (6a487db).
⚠️ Report is 28 commits behind head on develop.

Files with missing lines	Patch %	Lines
pybop/models/lithium_ion/utils.py	98.03%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #889      +/-   ##
===========================================
+ Coverage    89.41%   92.31%   +2.90%     
===========================================
  Files           63       72       +9     
  Lines         4857     5390     +533     
===========================================
+ Hits          4343     4976     +633     
+ Misses         514      414     -100     

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[brosaplanella]

I only have one comment: there's quite a lot of code duplication in set_initial_state (as far as I can tell). It might be better to have a pybop.lithium_ion.BaseModel with that method, from which all other classes are initialised.

I'm not too attached to this though, so if you think it's better to keep it as is, happy to approve.