set force_dtmin by default

[baggepinnen]

DelayDiffEq with high-order solvers sometimes struggle to simulate step responses, this PR adjusts a default value to the solver to accept that the dt becomes small around discontinuities in the input

[JuliaControlBot]

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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
❌ 0.046
✔️ 0.01

[codecov]

Codecov Report

Merging #690 (8f94ef7) into master (3889c07) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #690   +/-   ##
=======================================
  Coverage   87.45%   87.45%           
=======================================
  Files          34       34           
  Lines        3579     3579           
=======================================
  Hits         3130     3130           
  Misses        449      449           

Impacted Files	Coverage Δ
src/delay_systems.jl	88.88% <100.00%> (ø)

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