add method to `DSP.filt` and `filtfilt`

[codecov[bot]]

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

```diff @@ 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](https://app.codecov.io/gh/JuliaControl/ControlSystems.jl/pull/896?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaControl) | Coverage Δ | | |---|---|---| | [lib/ControlSystemsBase/src/dsp.jl](https://app.codecov.io/gh/JuliaControl/ControlSystems.jl/pull/896?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaControl#diff-bGliL0NvbnRyb2xTeXN0ZW1zQmFzZS9zcmMvZHNwLmps) | `0.00% <0.00%> (ø)` | |

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[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
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