accept symbolics in tf

[baggepinnen]

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[JuliaControlBot]

Something failed when generating plots. See the log at https://github.com/JuliaControl/ControlExamplePlots.jl/actions/runs/1643016516?check_suite_focus=true for more details.

[codecov]

Codecov Report

Merging #585 (03307ac) into master (27534ec) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #585   +/-   ##
=======================================
  Coverage   84.49%   84.49%           
=======================================
  Files          31       31           
  Lines        3179     3179           
=======================================
  Hits         2686     2686           
  Misses        493      493           

Impacted Files	Coverage Δ
src/types/SisoTfTypes/SisoRational.jl	80.00% <100.00%> (ø)

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