Add a WeightedCost

[NicolaCourtier]

Description

Add a WeightedCost class to combine costs and avoid duplicate problem evaluations where possible.

Issue reference

Fixes #327

Review

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[codecov]

Codecov Report

Attention: Patch coverage is 99.02913% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.10%. Comparing base (4f7eb59) to head (766bd16).
Report is 1 commits behind head on develop.

Files	Patch %	Lines
pybop/costs/_weighted_cost.py	98.33%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #329   +/-   ##
========================================
  Coverage    98.10%   98.10%           
========================================
  Files           43       44    +1     
  Lines         2632     2697   +65     
========================================
+ Hits          2582     2646   +64     
- Misses          50       51    +1     

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[BradyPlanden]

Hi @NicolaCourtier - this is looking great, I've made some suggestions in 327b to enable the design based costs. I'm happy for this to be merged ahead of that, or to have them merged together and then into develop. Looking forward to this functionality :)