Summary variables calculated only when called

[pipliggins]

Description

Fixes #4058

Type of change

• Optimization (back-end change that speeds up the code)

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

Attention: Patch coverage is 98.07692% with 2 lines in your changes missing coverage. Please review.

Project coverage is 99.21%. Comparing base (72c23ea) to head (b76d866).
Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/pybamm/solvers/summary_variable.py	97.70%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4621      +/-   ##
===========================================
- Coverage    99.21%   99.21%   -0.01%     
===========================================
  Files          302      303       +1     
  Lines        22858    22928      +70     
===========================================
+ Hits         22679    22747      +68     
- Misses         179      181       +2     

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[rtimms]

related: #4619