handle zero integral time

[baggepinnen]

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Difference	Reference Image	New Image
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[codecov]

Codecov Report

Merging #718 (26fd7f2) into master (3c14fd0) will increase coverage by 0.07%.
The diff coverage is 76.47%.

@@            Coverage Diff             @@
##           master     #718      +/-   ##
==========================================
+ Coverage   90.96%   91.04%   +0.07%     
==========================================
  Files          35       35              
  Lines        3742     3750       +8     
==========================================
+ Hits         3404     3414      +10     
+ Misses        338      336       -2     

Impacted Files	Coverage Δ
src/pid_design.jl	93.11% <76.47%> (+1.21%)	⬆️

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