optimize performance of sigma

[baggepinnen]

For MISO and SIMO systems, it is significantly faster to call norm rather than svdvals

[codecov]

Codecov Report

Merging #840 (0bae63a) into master (f62c25c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #840   +/-   ##
=======================================
  Coverage   97.14%   97.14%           
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  Files           4        4           
  Lines         315      315           
=======================================
  Hits          306      306           
  Misses          9        9           

[JuliaControlBot]

This is an automated message.
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
❌ 0.047
✔️ 0.011
✔️ 0.001
✔️ 0.001
✔️ 0.007
✔️ 0.003
✔️ 0.0
⚠️ 0.028
❌ 0.06
✔️ 0.0
⚠️ 0.017