Adding Composite Model Example

[teubert]

Adding the first example from 06 Combined Model. This is for composite models. Also added inputs, states, and outputs description to propeller load.

[github-actions]

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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[codecov-commenter]

Codecov Report

Merging #103 (1b884ee) into dev (b4637f7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #103   +/-   ##
=======================================
  Coverage   84.19%   84.19%           
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  Files          99       99           
  Lines       10183    10183           
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  Hits         8574     8574           
  Misses       1609     1609           

Files	Coverage Δ
src/progpy/models/propeller_load.py	100.00% <ø> (ø)

[kjjarvis]

This looks good. Just a few suggestions of additions that may add a little clarity

[kjjarvis]

Suggestion to add to the end, just so it's clear, "For this, we'll need to look at the DCMotor states and understand how they influence the PropellerLoad inputs".

[kjjarvis]

This looks great! I think these additions make everything flow smoothly. Great example