add constructors for workspaces

[baggepinnen]

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[JuliaControlBot]

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Plots were compared to references. 7/11 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
✔️ 0.0
❌ 0.043
❌ 0.046
✔️ 0.01

[codecov]

Codecov Report

Merging #684 (eb8dcab) into master (f57b89e) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #684      +/-   ##
==========================================
+ Coverage   87.42%   87.45%   +0.03%     
==========================================
  Files          34       34              
  Lines        3570     3579       +9     
==========================================
+ Hits         3121     3130       +9     
  Misses        449      449              

Impacted Files	Coverage Δ
src/freqresp.jl	91.03% <100.00%> (+0.17%)	⬆️
src/timeresp.jl	90.75% <100.00%> (+0.27%)	⬆️

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