feat: add xrfi_model_nonlinear_window

[steven-murray]

This adds xrfi_model_nonlinear_window which in tests gets MUCH better agreement in flagging RFI with Alan's code. The only remaining discrepancy is in the model fits themselves, which are only accurate to the kelvin-level at first, and sub-kelvin at higher iterations.

[codecov[bot]]

Codecov Report

Patch coverage: 3.57% and project coverage change: -1.55% :warning:

Comparison is base (d212c17) 85.20% compared to head (21f9de9) 83.66%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #124 +/- ## ========================================== - Coverage 85.20% 83.66% -1.55% ========================================== Files 15 15 Lines 2907 2963 +56 Branches 489 504 +15 ========================================== + Hits 2477 2479 +2 - Misses 302 356 +54 Partials 128 128 ``` | [Files Changed](https://app.codecov.io/gh/edges-collab/edges-cal/pull/124?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=edges-collab) | Coverage Δ | | |---|---|---| | [src/edges\_cal/xrfi.py](https://app.codecov.io/gh/edges-collab/edges-cal/pull/124?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=edges-collab#diff-c3JjL2VkZ2VzX2NhbC94cmZpLnB5) | `77.15% <3.57%> (-8.87%)` | :arrow_down: |

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[steven-murray]

Thanks @nmahesh1412. I think I'll have a look at one of the worse days and see if there's anything else going on there, before merging.