add correlogram method to `coherence`

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02% :tada:

Comparison is base (ae5fb17) 87.22% compared to head (ada3aff) 87.25%.

:exclamation: Current head ada3aff differs from pull request most recent head bb6b530. Consider uploading reports for the commit bb6b530 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #139 +/- ## ========================================== + Coverage 87.22% 87.25% +0.02% ========================================== Files 13 13 Lines 2310 2314 +4 ========================================== + Hits 2015 2019 +4 Misses 295 295 ``` | [Flag](https://app.codecov.io/gh/baggepinnen/ControlSystemIdentification.jl/pull/139/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/139/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson) | `87.25% <100.00%> (+0.02%)` | :arrow_up: | 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/139?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/139?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson#diff-c3JjL2ZyZC5qbA==) | `88.27% <100.00%> (+0.33%)` | :arrow_up: |

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