add 2DOF pid constructor

[baggepinnen]

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[JuliaControlBot]

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Plots were compared to references. 11/11 images have changed, see differences below.
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Difference	Reference Image	New Image
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❌ 0.06
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[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.79%. Comparing base (ceb9ff9) to head (048908f).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ControlSystemsBase/src/pid_design.jl	92.85%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #951      +/-   ##
==========================================
+ Coverage   92.74%   92.79%   +0.04%     
==========================================
  Files          41       41              
  Lines        4909     4928      +19     
==========================================
+ Hits         4553     4573      +20     
+ Misses        356      355       -1     

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