Add MultiFittingProblem class and example

[NicolaCourtier]

Description

Add a new Problem class that accepts multiple fitting problems and returns one combined problem for passing to a cost function. This PR builds on and should be merged after #359.

Issue reference

Fixes part of #238.

Review

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[codecov]

Codecov Report

Attention: Patch coverage is 98.82353% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.67%. Comparing base (a921c8f) to head (7eeeb00).
Report is 488 commits behind head on develop.

Files with missing lines	Patch %	Lines
pybop/models/base_model.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #364      +/-   ##
===========================================
- Coverage    98.68%   98.67%   -0.02%     
===========================================
  Files           46       47       +1     
  Lines         2971     3015      +44     
===========================================
+ Hits          2932     2975      +43     
- Misses          39       40       +1     

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[brosaplanella]

Overall looks good, nice work! See my comment on the parameters.

[BradyPlanden]

Thanks for this @NicolaCourtier. I think it's coming along well, I've made a few comments for you to look at. As a more general thought, do you see any reason this couldn't be a MultiProblem class that composes multiple instances ofFittingProblem or DesignProblem? Perhaps even combined instances of both (with an update to BaseOptimiser for minimising logic. This seems to be the logical next step, but we could do it now instead of refactoring this class later.

[NicolaCourtier]

Merge after #451

[BradyPlanden]

LGTM - thanks for the addition @NicolaCourtier!