Additions for #327 - Enable DesignCosts in WeightedCost

[BradyPlanden]

Description

This PR implements the following:

• Integrates DesignCost class into WeightedCost structure with support for sharing a single problem class
• Adds error checking for different problem class, with updates to optimiser.minimising
• Retains or updates variable names for clarity / alignment across the codebase
• Adds integration tests for WeightedCost with design and fitting problems

Issue reference

Fixes #327

Review

Before you mark your PR as ready for review, please ensure that you've considered the following:

• Updated the CHANGELOG.md in reverse chronological order (newest at the top) with a concise description of the changes, including the PR number.
• Noted any breaking changes, including details on how it might impact existing functionality.

Type of change

• New Feature: A non-breaking change that adds new functionality.
• Optimization: A code change that improves performance.
• Examples: A change to existing or additional examples.
• Bug Fix: A non-breaking change that addresses an issue.
• Documentation: Updates to documentation or new documentation for new features.
• Refactoring: Non-functional changes that improve the codebase.
• Style: Non-functional changes related to code style (formatting, naming, etc).
• Testing: Additional tests to improve coverage or confirm functionality.
• Other: (Insert description of change)

Key checklist:

• No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All unit tests pass: $ nox -s tests
• The documentation builds: $ nox -s doctest

You can run integration tests, unit tests, and doctests together at once, using $ nox -s quick.

Further checks:

• Code is well-commented, especially in complex or unclear areas.
• Added tests that prove my fix is effective or that my feature works.
• Checked that coverage remains or improves, and added tests if necessary to maintain or increase coverage.

Thank you for contributing to our project! Your efforts help us to deliver great software.

[codecov]

Codecov Report

Attention: Patch coverage is 99.13793% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.24%. Comparing base (8dca6d1) to head (1bedcd7).
Report is 1 commits behind head on develop.

Files	Patch %	Lines
pybop/problems/design_problem.py	94.11%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #413      +/-   ##
===========================================
- Coverage    98.28%   98.24%   -0.04%     
===========================================
  Files           46       46              
  Lines         2907     2910       +3     
===========================================
+ Hits          2857     2859       +2     
- Misses          50       51       +1     

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[NicolaCourtier]

I made some changes to clarify when each problem should be evaluated. Please check and see what you think! I also updated the output messages in spme_max_energy.py.

Happy to discuss further if you like, or proceed with this version.

[BradyPlanden]

Hi @NicolaCourtier - I've made a few updates, happy to discuss if you see anything worriesome, otherwise I'll merge :)

[NicolaCourtier]

Please just amend the attribute description, otherwise looks good to me, thanks!

[BradyPlanden]

Decreased coverage due to flaky test is fixed in #411. Merging as is.