add Mt_circles to nyquistplot

[baggepinnen]

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[JuliaControlBot]

This is an automated message.
Plots were compared to references. 8/11 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
❌ 0.048
✔️ 0.0
✔️ 0.001
✔️ 0.001
✔️ 0.0
❌ 0.043
❌ 0.059
✔️ 0.01

[codecov]

Codecov Report

Merging #700 (7aa0634) into master (3a92ba8) will increase coverage by 0.29%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master     #700      +/-   ##
==========================================
+ Coverage   87.58%   87.88%   +0.29%     
==========================================
  Files          35       35              
  Lines        3601     3614      +13     
==========================================
+ Hits         3154     3176      +22     
+ Misses        447      438       -9     

Impacted Files	Coverage Δ
src/plotting.jl	89.18% <92.85%> (+2.41%)	⬆️

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