add notes on solving for open loop from closed loop

[baggepinnen]

Sometimes it's easier to obtain a closed-loop transfer function, e.g., when performing system identification or frequency-response analysis of an unstable system. In such situations, one may want to solve for open-loop transfer functions given closed-loop ones. This PR adds a number of identities and a code example to the docs to assist in doing so.

[JuliaControlBot]

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Plots were compared to references. 1/11 images have changed, see differences below.
After pulling this PR, please update the reference images by creating a PR to ControlExamplePlots.jl here.

Difference	Reference Image	New Image
✔️ 0.008

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.42%. Comparing base (4052167) to head (0d8d0df).
Report is 2 commits behind head on master.

Additional details and impacted files

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##           master     #985   +/-   ##
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  Coverage   92.42%   92.42%           
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  Files          41       41           
  Lines        4990     4990           
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  Hits         4612     4612           
  Misses        378      378           

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