Build method external circuit

[aabills]

Description

This pull request refactors the external circuit submodel to use a single build method. It also sets up the code in BaseModel to use a build method, and to link coupled variables in the appropriate places.

Contributes to #3908

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

[codecov]

Codecov Report

Attention: Patch coverage is 99.27007% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.24%. Comparing base (bcdd0b5) to head (4214fb1).
Report is 7 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/pybamm/models/base_model.py	97.87%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4594      +/-   ##
===========================================
- Coverage    99.26%   99.24%   -0.03%     
===========================================
  Files          302      302              
  Lines        22889    22958      +69     
===========================================
+ Hits         22721    22784      +63     
- Misses         168      174       +6     

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