Update default init_soc for DesignProblem

[NicolaCourtier]

Description

Update the default value of the init_soc parameter of DesignProblem.

Issue reference

Fixes #421

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.52%. Comparing base (1bedcd7) to head (dcabe89).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #422      +/-   ##
===========================================
+ Coverage    98.24%   98.52%   +0.28%     
===========================================
  Files           46       46              
  Lines         2910     2923      +13     
===========================================
+ Hits          2859     2880      +21     
+ Misses          51       43       -8     

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[BradyPlanden]

Looks good - thanks! Just a few comments.

[BradyPlanden]

We should use the property for this private attribute instead of directly accessing it:

PyBOP/pybop/models/base_model.py

Line 662 in 8b09214

@property

(throughout PR)

[NicolaCourtier]

It would be easier for me to do this in #425 - where some of the logic regarding modification of the parameter sets is fixed. We can merge this PR into #425 and then into develop if you prefer.

[NicolaCourtier]

@BradyPlanden shall I merge this one into #425 or develop?

[BradyPlanden]

Either is fine with me, although if it merges into develop, the private attributes will need the be fixed in #435 before that can be merged.