allow plotting of only largest and smallest sigmas

[baggepinnen]

closes #846

[codecov[bot]]

Codecov Report

Merging #848 (071aec1) into master (2036570) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #848   +/-   ##
=======================================
  Coverage   92.72%   92.72%           
=======================================
  Files           5        5           
  Lines         330      330           
=======================================
  Hits          306      306           
  Misses         24       24           

[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