Add simplified battery model from tutorial

[teubert]

Added model that was built in the tutorial to the models subpackage. Added to test and documentation.

Also tested by updating PHM tutorial to use this model. Confirmed behavior is the same.

[github-actions[bot]]

Thank you for opening this PR. Each PR into dev requires a code review. For the code review, look at the following:

• [ ] Reviewer (someone other than author) should look for bugs, efficiency, readability, testing, and coverage in examples (if relevant).
• [ ] Ensure that each PR adding a new feature should include a test verifying that feature.
• [ ] All errors from static analysis must be resolved.
• [ ] Review the test coverage reports (if there is a change) - will be added as comment on PR if there is a change
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• [ ] Any added dependencies are included in requirements.txt, setup.py, and dev_guide.rst (this document)
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