add tools for counting multiple eigvals and adjust bode phase start

[baggepinnen]

The Bode plot phase curve should now start at the correct phase, even when this is outside of [-180, 180] degrees.

[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 78.78788% with 7 lines in your changes missing coverage. Please review.

Project coverage is 92.72%. Comparing base (fd44401) to head (96a92f8).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ControlSystemsBase/src/analysis.jl	94.11%	1 Missing ⚠️
lib/ControlSystemsBase/src/plotting.jl	83.33%	1 Missing ⚠️
lib/ControlSystemsBase/src/simplification.jl	66.66%	1 Missing ⚠️
lib/ControlSystemsBase/src/types/Lti.jl	0.00%	1 Missing ⚠️
lib/ControlSystemsBase/src/types/StateSpace.jl	0.00%	1 Missing ⚠️
...b/ControlSystemsBase/src/types/TransferFunction.jl	0.00%	1 Missing ⚠️
lib/ControlSystemsBase/src/types/conversion.jl	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #950      +/-   ##
==========================================
- Coverage   92.80%   92.72%   -0.08%     
==========================================
  Files          41       41              
  Lines        4879     4909      +30     
==========================================
+ Hits         4528     4552      +24     
- Misses        351      357       +6     

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