add concatenation for SimResult

[baggepinnen]

This is useful when performing multiple consecutive simulations, where some aspect of the simulation has been changed

[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
❌ 0.047
✔️ 0.011
✔️ 0.001
✔️ 0.001
✔️ 0.007
✔️ 0.003
✔️ 0.0
⚠️ 0.028
❌ 0.06
✔️ 0.0
⚠️ 0.017

[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
❌ 0.047
✔️ 0.011
✔️ 0.001
✔️ 0.001
✔️ 0.007
✔️ 0.003
✔️ 0.0
⚠️ 0.028
❌ 0.06
✔️ 0.0
⚠️ 0.017

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.89%. Comparing base (b39d0e8) to head (add7d76).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ControlSystemsBase/src/timeresp.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #945      +/-   ##
==========================================
+ Coverage   92.78%   92.89%   +0.11%     
==========================================
  Files          41       41              
  Lines        4864     4872       +8     
==========================================
+ Hits         4513     4526      +13     
+ Misses        351      346       -5     

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