make `balance_statespace` work for more system types

[baggepinnen]

closes #835

[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
:x: 0.047
:heavy_check_mark: 0.011
:heavy_check_mark: 0.001
:heavy_check_mark: 0.001
:heavy_check_mark: 0.007
:heavy_check_mark: 0.003
:heavy_check_mark: 0.0
:warning: 0.028
:x: 0.06
:heavy_check_mark: 0.0
:warning: 0.017

[codecov[bot]]

Codecov Report

Merging #837 (3c7632f) into master (ba2713d) will increase coverage by 1.58%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #837      +/-   ##
==========================================
+ Coverage   95.55%   97.14%   +1.58%     
==========================================
  Files           4        4              
  Lines         315      315              
==========================================
+ Hits          301      306       +5     
+ Misses         14        9       -5     

see 1 file with indirect coverage changes