Remove matrix size from StateSpace types

[baggepinnen]

Solves #399 and supersedes #400

[olof3]

Looks good, great to get rid of those fields. There were a lot of other fixes in #410, what about those?

[baggepinnen]

I accidentally included tons of unrelated commits from my working branch. They might make their way into master eventually, but I have a few PRs waiting to be merged before I can submit the rest of the changes.

[olof3]

Looked like there was a lot of useful stuff there :)

[baggepinnen]

Hmm, the failing test only fails on julia v1.5, not on julia v1.0 or nightly
https://github.com/JuliaControl/ControlSystems.jl/pull/411/checks?check_run_id=1714851690#step:6:251
I'll deactivate the test temporarily, it's a type-stability test and I do't believe the spurious failure on v1.5 but not on v1.6 is anything to worry about.

[codecov]

Codecov Report

Merging #411 (550fd02) into master (6c54a25) will decrease coverage by 5.30%.
The diff coverage is 96.87%.

@@            Coverage Diff             @@
##           master     #411      +/-   ##
==========================================
- Coverage   82.40%   77.09%   -5.31%     
==========================================
  Files          31       31              
  Lines        2824     2855      +31     
==========================================
- Hits         2327     2201     -126     
- Misses        497      654     +157     

Impacted Files	Coverage Δ
src/types/StateSpace.jl	90.16% <96.87%> (-0.32%)	⬇️
src/pid_design.jl	32.57% <0.00%> (-45.38%)	⬇️
src/discrete.jl	40.54% <0.00%> (-43.72%)	⬇️
src/types/SisoTf.jl	25.00% <0.00%> (-25.00%)	⬇️
src/types/SisoTfTypes/SisoZpk.jl	60.00% <0.00%> (-21.11%)	⬇️
src/types/Lti.jl	50.00% <0.00%> (-7.15%)	⬇️
src/freqresp.jl	88.00% <0.00%> (-3.90%)	⬇️
src/utilities.jl	80.21% <0.00%> (-2.58%)	⬇️
src/timeresp.jl	91.33% <0.00%> (-2.21%)	⬇️
src/types/SisoTfTypes/SisoRational.jl	72.58% <0.00%> (-1.62%)	⬇️
... and 3 more

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