usability improvements for multiplication with scalars

[baggepinnen]

I've verified that this aligns with how multiplication works in matlab, in particular

@test_throws ErrorException C_111 * C_222

is now replaced by

@test C_111*C_222 == ss([-5 0 2 0; 0 -5 0 2; 0 0 -5 -3; 0 0 2 -9], [0 0; 0 0; 1 0; 0 2], [3 0 0 0; 0 3 0 0], 0)

which is the same result as matlab returns.
This allows, e.g.,

P*I(2) # for 1x1 P

which makes systems much easier to work with.

Here's also a fallback for HeteroStateSpace that makes it convert to a regular statespace if it's promoted with a matrix without knowing how the matrix is to be used later.

[JuliaControlBot]

Something failed when generating plots. See the log at https://github.com/JuliaControl/ControlExamplePlots.jl/actions/runs/1606936042?check_suite_focus=true for more details.

[codecov]

Codecov Report

Merging #581 (d4c94ae) into master (744e263) will decrease coverage by 0.05%.
The diff coverage is 76.47%.

@@            Coverage Diff             @@
##           master     #581      +/-   ##
==========================================
- Coverage   84.54%   84.49%   -0.06%     
==========================================
  Files          31       31              
  Lines        3165     3179      +14     
==========================================
+ Hits         2676     2686      +10     
- Misses        489      493       +4     

Impacted Files	Coverage Δ
src/types/promotion.jl	30.00% <0.00%> (-5.30%)	⬇️
src/types/StateSpace.jl	87.55% <91.66%> (-0.88%)	⬇️
src/analysis.jl	87.17% <100.00%> (+0.08%)	⬆️
src/types/Lti.jl	60.00% <0.00%> (+4.44%)	⬆️

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