Always draw up to Nyquist frequency for discrete systems

[JuliaControlBot]

This is an automated message. 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. Difference	Reference Image	New Image
:x: 0.047
:heavy_check_mark: 0.011
:heavy_check_mark: 0.001
:heavy_check_mark: 0.001
:heavy_check_mark: 0.007
:heavy_check_mark: 0.003
:heavy_check_mark: 0.0
:warning: 0.028
:x: 0.06
:heavy_check_mark: 0.0
:warning: 0.017

[codecov[bot]]

Codecov Report

Merging #860 (2e57172) into master (a047432) will increase coverage by 90.90%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #860       +/-   ##
===========================================
+ Coverage    0.30%   91.21%   +90.90%     
===========================================
  Files           5        5               
  Lines         329      330        +1     
===========================================
+ Hits            1      301      +300     
+ Misses        328       29      -299     

see 3 files with indirect coverage changes