Split Problem into Base, Fitting and Design

[NicolaCourtier]

An initial PR to restructure Problem ahead of adding a design optimisation example.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (5225cfc) 90.57% compared to head (f46fa6f) 92.24%.
Report is 22 commits behind head on develop.

Files	Patch %	Lines
pybop/_problem.py	97.50%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #125      +/-   ##
===========================================
+ Coverage    90.57%   92.24%   +1.67%     
===========================================
  Files           18       30      +12     
  Lines          647     1057     +410     
===========================================
+ Hits           586      975     +389     
- Misses          61       82      +21     

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[BradyPlanden]

Hi @NicolaCourtier, nice one! How do you feel about changing FittingProblem to FitProblem?

[BradyPlanden]

It's looking good @NicolaCourtier! I've left few comments above, can you update the CHANGELOG.md as well?

[NicolaCourtier]

Thanks for the review @BradyPlanden! I've acted on your feedback so it would be great if you could take another look at the latest version.

[BradyPlanden]

Nice one @NicolaCourtier, thanks for the great contribution! I left a few comments, but looks really good overall!