Ensure costs return evaluations

[NicolaCourtier]

Description

Align the cost and problem evaluate methods by ensuring that the costs also return a pybop.Evaluation.

To-do:

• enable a list of solutions and corresponding inputs to be evaluated in a single call
• update the comparing_cost_functions.ipynb example notebook
• (future goal) enable costs to be evaluated in parallel

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #).

Important checks:

Please confirm the following before marking the PR as ready for review:

• 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 tests pass: nox -s tests
• The documentation builds: nox -s doctest
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

❌ Patch coverage is 97.29730% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.89%. Comparing base (4628a65) to head (47826e9).
⚠️ Report is 50 commits behind head on develop.

Files with missing lines	Patch %	Lines
pybop/problems/problem.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #942      +/-   ##
===========================================
+ Coverage    89.41%   90.89%   +1.47%     
===========================================
  Files           63       73      +10     
  Lines         4857     5690     +833     
===========================================
+ Hits          4343     5172     +829     
- Misses         514      518       +4     

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