CoupledVariable

[aabills]

Description

Adds CoupledVariable from #3908. Does not really do integration work which will be required to use this for submodels.

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.

• [x] 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:

• [x] 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)
• [x] All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• [x] 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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.42%. Comparing base (01a7c08) to head (c259bb0). Report is 7 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #4556 +/- ## ======================================== Coverage 99.42% 99.42% ======================================== Files 299 300 +1 Lines 22736 22774 +38 ======================================== + Hits 22605 22643 +38 Misses 131 131 ```

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