marginplot to support frequency display in Hertz

[baggepinnen]

close #827

• Added hz parameter to marginplot to allow x-axis display in Hertz while treating input frequencies as rad/s.

[JuliaControlBot]

This is an automated message.
Plots were compared to references. 4/11 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
✔️ 0.0
✔️ 0.0
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✔️ 0.0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.28%. Comparing base (f04916f) to head (8bcc023).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1017   +/-   ##
=======================================
  Coverage   92.28%   92.28%           
=======================================
  Files          41       41           
  Lines        5159     5162    +3     
=======================================
+ Hits         4761     4764    +3     
  Misses        398      398           

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