add option to draw an M circle in the Nyquist plot

[baggepinnen]

[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
:x: 0.047
:heavy_check_mark: 0.011
:heavy_check_mark: 0.001
:heavy_check_mark: 0.001
:heavy_check_mark: 0.007
:heavy_check_mark: 0.003
:heavy_check_mark: 0.0
:warning: 0.028
:x: 0.06
:heavy_check_mark: 0.0
:warning: 0.017

[codecov[bot]]

Codecov Report

Merging #856 (d343612) into master (42e3955) will increase coverage by 1.51%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #856      +/-   ##
==========================================
+ Coverage   91.21%   92.72%   +1.51%     
==========================================
  Files           5        5              
  Lines         330      330              
==========================================
+ Hits          301      306       +5     
+ Misses         29       24       -5     

see 1 file with indirect coverage changes