widen the scope of ss*mat method

[baggepinnen]

so that all isdiag matrices behave the same

[JuliaControlBot]

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Plots were compared to references. 11/11 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
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❌ 0.06
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.80%. Comparing base (9f0bcff) to head (8e9aeae).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #949   +/-   ##
=======================================
  Coverage   92.80%   92.80%           
=======================================
  Files          41       41           
  Lines        4879     4879           
=======================================
  Hits         4528     4528           
  Misses        351      351           

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