Add polarized to integrability

[giacomomagni]

Add possibility to have polarized xfx FK tables.

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (f95e751) 29.17% compared to head (897dde6) 29.12%.

Files	Patch %	Lines
src/pinefarm/external/integrability.py	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #57 +/- ## ========================================== - Coverage 29.17% 29.12% -0.05% ========================================== Files 24 24 Lines 1258 1260 +2 ========================================== Hits 367 367 - Misses 891 893 +2 ``` | [Flag](https://app.codecov.io/gh/NNPDF/pinefarm/pull/57/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NNPDF) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/NNPDF/pinefarm/pull/57/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NNPDF) | `29.12% <0.00%> (-0.05%)` | :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=NNPDF#carryforward-flags-in-the-pull-request-comment) to find out more.

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[giacomomagni]

I'm reopening this PR as I think we should impose integrability on \Delta f(x) and not x \Delta f(x) as currently implemented. See #58