Add support for pure D systems in lsim function and corresponding tests

[baggepinnen]

closes #1024

[JuliaControlBot]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.20%. Comparing base (3e71154) to head (f435a3e).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1025      +/-   ##
==========================================
+ Coverage   92.17%   92.20%   +0.03%     
==========================================
  Files          42       42              
  Lines        5535     5558      +23     
==========================================
+ Hits         5102     5125      +23     
  Misses        433      433              

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