feat: Discrete Time Sum and Explicit Time Integral support for output variables

[martinjrobins]

Description

Allows pybamm.DiscreteTimeSum and pybamm.ExplicitTimeIntegral variables to work with the output_varables solver option

Fixes #5063

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: 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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.12%. Comparing base (6dc7fd0) to head (aa7288c).
⚠️ Report is 168 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5071      +/-   ##
===========================================
- Coverage    99.12%   99.12%   -0.01%     
===========================================
  Files          305      304       -1     
  Lines        23557    23573      +16     
===========================================
+ Hits         23351    23366      +15     
- Misses         206      207       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[BradyPlanden]

Looks great, thanks @martinjrobins. I can also confirm this works as expected on the PyBOP side.