relax some conversion signatures

[baggepinnen]

helps named statespace from RobustAndOptimalControl

[codecov[bot]]

Codecov Report

Merging #805 (40ff318) into master (dfabb60) will increase coverage by 97.14%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #805       +/-   ##
===========================================
+ Coverage    0.00%   97.14%   +97.14%     
===========================================
  Files           3        4        +1     
  Lines         313      315        +2     
===========================================
+ Hits            0      306      +306     
+ Misses        313        9      -304     

Impacted Files	Coverage Δ
src/ControlSystems.jl	100.00% <0.00%> (ø)
src/simulators.jl	87.50% <0.00%> (+87.50%)	:arrow_up:
src/timeresp.jl	97.22% <0.00%> (+97.22%)	:arrow_up:
src/root_locus.jl	97.77% <0.00%> (+97.77%)	:arrow_up:

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[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: 0.048
:heavy_check_mark: 0.011
:heavy_check_mark: 0.001
:heavy_check_mark: 0.001
:heavy_check_mark: 0.006
:heavy_check_mark: 0.001
:heavy_check_mark: 0.0
:warning: 0.028
:x: 0.06
:heavy_check_mark: 0.0
:warning: 0.017