Add interface to DSP

[codecov[bot]]

Codecov Report

Merging #843 (3f9726b) into master (cc5dcda) will decrease coverage by 4.42%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #843      +/-   ##
==========================================
- Coverage   97.14%   92.72%   -4.42%     
==========================================
  Files           4        5       +1     
  Lines         315      330      +15     
==========================================
  Hits          306      306              
- Misses          9       24      +15     

Impacted Files	Coverage Δ
src/dsp.jl	0.00% <0.00%> (ø)

[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