Add Aqua.jl tests

[baggepinnen]

https://github.com/JuliaTesting/Aqua.jl

[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
❌ 0.046
✔️ 0.01

[codecov]

Codecov Report

Merging #691 (8554f63) into master (8582366) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #691   +/-   ##
=======================================
  Coverage   87.45%   87.45%           
=======================================
  Files          34       34           
  Lines        3579     3579           
=======================================
  Hits         3130     3130           
  Misses        449      449           

Impacted Files	Coverage Δ
src/ControlSystems.jl	100.00% <ø> (ø)
src/hammerstein_weiner.jl	92.12% <ø> (ø)
src/plotting.jl	86.76% <ø> (ø)
src/types/TimeEvolution.jl	83.87% <100.00%> (ø)
src/types/zpk.jl	97.43% <100.00%> (ø)

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