Add ECM variables

[TomTranter]

Description

Some extra variables for the ecm model based on using pybamm in an external equivalent circuit model

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: $ 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 #1204 into develop will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1204   +/-   ##
========================================
  Coverage    97.93%   97.93%           
========================================
  Files          247      247           
  Lines        14024    14036   +12     
========================================
+ Hits         13734    13746   +12     
  Misses         290      290           

Impacted Files	Coverage Δ
...m/models/full_battery_models/base_battery_model.py	99.67% <100.00%> (+0.01%)	⬆️

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[rtimms]

looks good thanks, would you be able to reorganize this a little so that all the ECM variables are defined together and just add a short comment explaining them (its not obvious as we have no other ECM stuff in pybamm)

[TomTranter]

@rtimms why failing on mac? I'm not authorized to merge this so please do so if you're ready. Thanks

[valentinsulzer]

Everything is failing on mac right now. We need to wait for github to fix it