add method to DSP.filt and filtfilt

[baggepinnen]

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[codecov]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (b3ffc72) 31.57% compared to head (002a315) 31.47%.
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #896      +/-   ##
==========================================
- Coverage   31.57%   31.47%   -0.10%     
==========================================
  Files          43       43              
  Lines        4624     4638      +14     
==========================================
  Hits         1460     1460              
- Misses       3164     3178      +14     

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

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[JuliaControlBot]

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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.

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