#878 reformat external circuit

[valentinsulzer]

Description

Reformat external circuit submodels to get rid of unnecessary abstraction

Fixes #878

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: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --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

Merging #879 into develop will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #879      +/-   ##
===========================================
- Coverage    97.79%   97.78%   -0.02%     
===========================================
  Files          210      210              
  Lines        10585    10580       -5     
===========================================
- Hits         10352    10346       -6     
- Misses         233      234       +1     

Impacted Files	Coverage Δ
...ubmodels/external_circuit/base_external_circuit.py	100.00% <ø> (ø)
pybamm/parameters/parameter_values.py	100.00% <ø> (ø)
...ernal_circuit/function_control_external_circuit.py	97.77% <100.00%> (-2.23%)	⬇️

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[rtimms]

thanks