Adding decomposed FoKL GPs into PyBOP

[derek-slack]

Description

This push adds the ability to create FoKL GPs as parameter functions of the state of the cell.

Fixes # 672

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

[derek-slack]

I fixed the issue where the end user needed to use indices to call function inputs instead of strings:

User can now select from the set of passed children to the function by referencing the string in the Arguments options i.e:
GP_options = {'Arguments': ['Concentration in electrolyte [mol.m-3]']}
or similarly for division arguments:
GP_options = {'Arguments': [['Concentration in electrode [mol.m-3]',['Maximum concentration in electrode [mol.m-3]']]}

[codecov]

Codecov Report

❌ Patch coverage is 97.66355% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.04%. Comparing base (3171a07) to head (ce1f4e7).
⚠️ Report is 20 commits behind head on develop.

Files with missing lines	Patch %	Lines
pybop/parameters/gp_parameter.py	97.65%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #978      +/-   ##
===========================================
+ Coverage    92.13%   93.04%   +0.91%     
===========================================
  Files           74       85      +11     
  Lines         5799     6991    +1192     
===========================================
+ Hits          5343     6505    +1162     
- Misses         456      486      +30     

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