use "built-in" blockdiag

[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
✔️ 0.003
✔️ 0.003
✔️ 0.0
✔️ 0.001
✔️ 0.0
✔️ 0.0
❌ 0.042
✔️ 0.006
✔️ 0.011

[codecov]

Codecov Report

Merging #627 (7184b82) into master (9455a54) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #627      +/-   ##
==========================================
- Coverage   86.75%   86.69%   -0.06%     
==========================================
  Files          31       31              
  Lines        3253     3239      -14     
==========================================
- Hits         2822     2808      -14     
  Misses        431      431              

Impacted Files	Coverage Δ
src/connections.jl	92.68% <100.00%> (-0.58%)	⬇️

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