make feedback2dof work for any system type

[baggepinnen]

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[JuliaControlBot]

This is an automated message.
Plots were compared to references. 11/11 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
❌ 0.047
✔️ 0.011
✔️ 0.001
✔️ 0.001
✔️ 0.007
✔️ 0.003
✔️ 0.0
⚠️ 0.028
❌ 0.06
✔️ 0.0
⚠️ 0.017

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.81%. Comparing base (5982cc3) to head (a7cd0f0).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ControlSystemsBase/src/connections.jl	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #952      +/-   ##
==========================================
+ Coverage   92.79%   92.81%   +0.01%     
==========================================
  Files          41       41              
  Lines        4928     4938      +10     
==========================================
+ Hits         4573     4583      +10     
  Misses        355      355              

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