Add standard variables to Thevenin() class

[BradyPlanden]

Description

This fixes #3129.

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 (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
• The documentation builds: $ python run-tests.py --doctest

You can run unit and doctests together 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

Patch coverage: 100.00% and no project coverage change.

Comparison is base (10f9f46) 99.71% compared to head (9f2505e) 99.71%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3143   +/-   ##
========================================
  Coverage    99.71%   99.71%           
========================================
  Files          248      248           
  Lines        18748    18751    +3     
========================================
+ Hits         18694    18697    +3     
  Misses          54       54           

Impacted Files	Coverage Δ
...full_battery_models/equivalent_circuit/thevenin.py	100.00% <100.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[rtimms]

thanks @BradyPlanden !