Loss function solver

[martinjrobins]

Description

Adds a LossSolver class that takes a pybamm simulation instance and constructs a loss function which is encoded as a pybamm.Symbol instance over a set duration provided by the user. The loss function can either contain a pybamm.ExplicitTimeIntegral, in which case the loss function is a continuous integral over the simulation trajectory, or a pybamm.DiscreteTimeSum, in which case the loss function is a discrete sum defined at a series of times.

Under the hood this uses pydiffsol (https://github.com/alexallmont/pydiffsol) as the solver, so any solver instance on the Simulation is ignored, except for the predict method, which uses the original Simulation solver. This is mainly provided as a convenience method.

Fixes #5559

Type of change

Add an entry under # [Unreleased] in CHANGELOG.md, in one of ## Breaking changes, ## Deprecated, ## Features, or ## Bug fixes (include PR number; breaking and deprecation entries need a one-line migration note). Internal-only PRs — refactor, docs, CI, tests — can skip this. See RELEASE.md for the full policy.

Important checks:

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

• No style issues: nox -s pre-commit
• All tests pass: nox -s tests
• The documentation builds: nox -s doctests
• 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 98.22222% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.12%. Comparing base (9e97e94) to head (a93876a).

Files with missing lines	Patch %	Lines
...mm/src/pybamm/expression_tree/operations/diffsl.py	90.47%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5652      +/-   ##
==========================================
- Coverage   98.12%   98.12%   -0.01%     
==========================================
  Files         339      340       +1     
  Lines       32079    32284     +205     
==========================================
+ Hits        31477    31678     +201     
- Misses        602      606       +4     

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