add backward euler discretization

[baggepinnen]

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[JuliaControlBot]

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[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.80%. Comparing base (4c404ef) to head (e28eafc).
Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ControlSystemsBase/src/discrete.jl	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #967      +/-   ##
==========================================
+ Coverage   92.78%   92.80%   +0.01%     
==========================================
  Files          41       41              
  Lines        4975     4987      +12     
==========================================
+ Hits         4616     4628      +12     
  Misses        359      359              

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