add discretization of cost and covariance matrices

[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 #862 (92010a5) into master (7407550) will increase coverage by 1.51%. The diff coverage is n/a.

:exclamation: Current head 92010a5 differs from pull request most recent head e6a7844. Consider uploading reports for the commit e6a7844 to get more accurate results

@@            Coverage Diff             @@
##           master     #862      +/-   ##
==========================================
+ Coverage   91.21%   92.72%   +1.51%     
==========================================
  Files           5        5              
  Lines         330      330              
==========================================
+ Hits          301      306       +5     
+ Misses         29       24       -5     

see 1 file with indirect coverage changes