add functions relating Ms peak to gain and phase margins

[baggepinnen]

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[JuliaControlBot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.52%. Comparing base (20ed7ea) to head (3136191).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #996      +/-   ##
==========================================
+ Coverage   92.51%   92.52%   +0.01%     
==========================================
  Files          41       41              
  Lines        5076     5083       +7     
==========================================
+ Hits         4696     4703       +7     
  Misses        380      380              

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