add welch option for `tfest`

[baggepinnen]

@fibonatic

[codecov[bot]]

Codecov Report

Patch coverage: 85.71% and project coverage change: -0.03% :warning:

Comparison is base (a1b529c) 87.38% compared to head (364daaf) 87.35%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #142 +/- ## ========================================== - Coverage 87.38% 87.35% -0.03% ========================================== Files 13 13 Lines 2314 2318 +4 ========================================== + Hits 2022 2025 +3 - Misses 292 293 +1 ``` | [Flag](https://app.codecov.io/gh/baggepinnen/ControlSystemIdentification.jl/pull/142/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/baggepinnen/ControlSystemIdentification.jl/pull/142/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson) | `87.35% <85.71%> (-0.03%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/baggepinnen/ControlSystemIdentification.jl/pull/142?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson) | Coverage Δ | | |---|---|---| | [src/frd.jl](https://app.codecov.io/gh/baggepinnen/ControlSystemIdentification.jl/pull/142?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson#diff-c3JjL2ZyZC5qbA==) | `87.91% <85.71%> (-0.36%)` | :arrow_down: |

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