export balance_statespace

[baggepinnen]

• add test
• fix docstring

[JuliaControlBot]

This is an automated message.
Plots were compared to references. 3/11 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
✔️ 0.001
❌ 0.043
✔️ 0.002

[codecov]

Codecov Report

Merging #609 (2d4eb57) into master (04cef38) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #609   +/-   ##
=======================================
  Coverage   86.81%   86.81%           
=======================================
  Files          31       31           
  Lines        3239     3239           
=======================================
  Hits         2812     2812           
  Misses        427      427           

Impacted Files	Coverage Δ
src/ControlSystems.jl	100.00% <ø> (ø)
src/types/conversion.jl	96.29% <ø> (ø)

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