simplify expression of cooling terms in x-lumped thermal models

[rtimms]

Description

Rewrite the cooling terms in the x-lumped and lumped thermal models in terms of fluxes. IMO it is easier to follow the code now. The equations are unchanged, just expressed differently.

Fixes # (issue)

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

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 tests pass: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

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Codecov Report

All modified lines are covered by tests ✅

Comparison is base (bd5e240) 99.58% compared to head (ce8e56b) 99.58%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3439   +/-   ##
========================================
  Coverage    99.58%   99.58%           
========================================
  Files          256      256           
  Lines        19998    20004    +6     
========================================
+ Hits         19915    19921    +6     
  Misses          83       83           

Files	Coverage Δ
pybamm/models/submodels/thermal/lumped.py	100.00% <100.00%> (ø)
...mal/pouch_cell/pouch_cell_1D_current_collectors.py	100.00% <100.00%> (ø)
...mal/pouch_cell/pouch_cell_2D_current_collectors.py	100.00% <100.00%> (ø)

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[brosaplanella]

Looks great, thanks! Not sure if this needs a CHANGELOG line, I leave it up to you. Let's wait to merge until the wheels are fixed though, so we make it easier for Saransh to cherrypick the commits.

[Saransh-cpp]

Just saw the comment, sorry for the delay. Everything is fixed now, feel free to merge!

[brosaplanella]

@rtimms if you are happy without a CHANGELOG line feel free to merge this