add discrete-time delay constructor

[baggepinnen]

closes #773

[codecov]

Codecov Report

Merging #774 (c0320a0) into master (a3ae694) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #774   +/-   ##
=======================================
  Coverage   95.23%   95.23%           
=======================================
  Files           4        4           
  Lines         294      294           
=======================================
  Hits          280      280           
  Misses         14       14           

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[JuliaControlBot]

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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.

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