provide signal names to SimResult plot

[baggepinnen]

using Plots
A = [-10 2; 3 -40]
B = [5 6; 7 8]
C = [9 10; 11 12]
G = ss(A, B, C, 0)
step_response = step(G, 10.0)
plot(step_response, title="", xlabel="Time (s)", lw=2, legend=:topright, input_names=["a", "b"], output_names=["c", "d"])

[JuliaControlBot]

This is an automated message.
Plots were compared to references. 1/11 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
✔️ 0.008

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.44%. Comparing base (8b43e08) to head (de72455).

Files with missing lines	Patch %	Lines
lib/ControlSystemsBase/src/plotting.jl	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #991   +/-   ##
=======================================
  Coverage   92.44%   92.44%           
=======================================
  Files          41       41           
  Lines        5006     5006           
=======================================
  Hits         4628     4628           
  Misses        378      378           

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