remove ny,nu fields from TransferFunction

[baggepinnen]

Closes #399

[JuliaControlBot]

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Plots were compared to references. 9/11 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
✔️ 0.003
✔️ 0.003
✔️ 0.0
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✔️ 0.0
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❌ 0.042
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✔️ 0.011

[JuliaControlBot]

This is an automated message.
Plots were compared to references. 9/11 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
✔️ 0.003
✔️ 0.003
✔️ 0.0
✔️ 0.001
✔️ 0.0
✔️ 0.0
❌ 0.042
✔️ 0.006
✔️ 0.011

[albheim]

Realised I approved without checking if it passed... But the changes seem reasonable to me so as long as the expected tests pass I think it is all good :)

[codecov]

Codecov Report

Merging #624 (54d5df8) into master (9455a54) will decrease coverage by 0.11%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #624      +/-   ##
==========================================
- Coverage   86.75%   86.63%   -0.12%     
==========================================
  Files          31       31              
  Lines        3253     3255       +2     
==========================================
- Hits         2822     2820       -2     
- Misses        431      435       +4     

Impacted Files	Coverage Δ
src/types/Lti.jl	55.55% <ø> (-4.45%)	⬇️
src/types/TransferFunction.jl	89.56% <82.35%> (-1.70%)	⬇️
src/types/PartionedStateSpace.jl	94.23% <100.00%> (+0.11%)	⬆️
src/types/StateSpace.jl	87.44% <100.00%> (+0.10%)	⬆️
src/connections.jl	92.68% <0.00%> (-0.58%)	⬇️
src/types/conversion.jl	97.03% <0.00%> (+0.74%)	⬆️

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