fix and test type stability in system concatenation

[baggepinnen]

Also reduces some allocations

[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
✔️ 0.001
✔️ 0.001
✔️ 0.0
✔️ 0.0
❌ 0.042
✔️ 0.006
✔️ 0.011

[codecov]

Codecov Report

Merging #643 (cc4814e) into master (4c4c205) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #643   +/-   ##
=======================================
  Coverage   86.75%   86.75%           
=======================================
  Files          31       31           
  Lines        3253     3253           
=======================================
  Hits         2822     2822           
  Misses        431      431           

Impacted Files	Coverage Δ
src/connections.jl	92.68% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4c4c205...cc4814e. Read the comment docs.