add Mt_lines option to gofplot

[baggepinnen]

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[JuliaControlBot]

This is an automated message.
Plots were compared to references. 9/11 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
✔️ 0.001
✔️ 0.001
✔️ 0.001
✔️ 0.0
❌ 0.034
❌ 0.059
✔️ 0.0
✔️ 0.01

[codecov]

Codecov Report

Merging #737 (a5c00a1) into master (5f0fc84) will increase coverage by 0.00%.
The diff coverage is 83.33%.

@@           Coverage Diff           @@
##           master     #737   +/-   ##
=======================================
  Coverage   91.85%   91.85%           
=======================================
  Files          37       37           
  Lines        3976     3978    +2     
=======================================
+ Hits         3652     3654    +2     
  Misses        324      324           

Impacted Files	Coverage Δ
src/plotting.jl	92.50% <80.00%> (+0.03%)	⬆️
src/pid_design.jl	94.28% <100.00%> (ø)

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