Release v26.4.0

[rtimms]

Summary

Release v26.4.0 — version bump and CHANGELOG update.

See the v26.4.0 section of CHANGELOG.md for the full list of features and bug fixes included (7 features, 3 bug fixes).

Test plan

• CI passes on this PR
• After merge, tag v26.4.0 from main triggers publish_pypi.yml
• pip install pybamm==26.4.0 succeeds

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.29%. Comparing base (4f78ef0) to head (e2605f9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5455   +/-   ##
=======================================
  Coverage   98.29%   98.29%           
=======================================
  Files         337      337           
  Lines       30958    30958           
=======================================
  Hits        30429    30429           
  Misses        529      529           

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[rtimms]

let's wait on #5456 and #5457 @MarcBerliner