Add time-scaling function.

[baggepinnen]

The added function time_scale changes the time unit of a system, for example, from seconds to microseconds. Modeling on the correct time scale is important for numerical performance, in particular for continuous-time simulations.

Closes #702

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Plots were compared to references. 8/11 images have changed, see differences below.
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Difference	Reference Image	New Image
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❌ 0.043
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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

[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 #703 (c45c7ef) into master (308434d) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #703      +/-   ##
==========================================
+ Coverage   87.88%   87.91%   +0.03%     
==========================================
  Files          35       35              
  Lines        3614     3624      +10     
==========================================
+ Hits         3176     3186      +10     
  Misses        438      438              

Impacted Files	Coverage Δ
src/ControlSystems.jl	100.00% <ø> (ø)
src/hammerstein_weiner.jl	92.12% <ø> (ø)
src/matrix_comps.jl	93.28% <100.00%> (+0.24%)	⬆️

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