add fast path for kd=0 in pid_2dof

[baggepinnen]

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[JuliaControlBot]

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[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.79%. Comparing base (82a5014) to head (7068fbd).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ControlSystemsBase/src/pid_design.jl	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #957      +/-   ##
==========================================
- Coverage   92.79%   92.79%   -0.01%     
==========================================
  Files          41       41              
  Lines        4958     4966       +8     
==========================================
+ Hits         4601     4608       +7     
- Misses        357      358       +1     

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