Fix parameter set logic and allow FittingProblem to take init_ocv rather than init_soc

[NicolaCourtier]

Description

Remove the init_soc option from BaseProblem, keep the init_soc option for DesignProblem and replace it with an init_ocv option for FittingProblem and Observers. This change is to ensure consistency in problems with parameters that affect the calculation of the initial concentrations from SOC.

Issue reference

Fixes #424

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[codecov]

Codecov Report

Attention: Patch coverage is 99.45355% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.68%. Comparing base (d68a70d) to head (db09455).
Report is 2 commits behind head on develop.

Files	Patch %	Lines
pybop/problems/fitting_problem.py	96.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #425      +/-   ##
===========================================
+ Coverage    98.51%   98.68%   +0.16%     
===========================================
  Files           46       46              
  Lines         2960     2961       +1     
===========================================
+ Hits          2916     2922       +6     
+ Misses          44       39       -5     

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[NicolaCourtier]

TODO: Re-run all notebooks after #428 and check output.

[MarkBlyth]

I'm happy with all this. Since initial_state can be an OCV or SOC, it won't break any of the functionality needed for typical cell-cycling tests.

[BradyPlanden]

Looks good, thanks for the additions @NicolaCourtier!