Feat/frac diff

[ngriffiths13]

Added frac diff functionality and tests ensuring it matches the pandas implementation in AFML

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.23%. Comparing base (98f68e1) to head (63f27a8). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #10 +/- ## ========================================== + Coverage 94.00% 94.23% +0.23% ========================================== Files 6 7 +1 Lines 150 156 +6 ========================================== + Hits 141 147 +6 Misses 9 9 ```

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[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #10 will not alter performance

_{Comparing feat/frac-diff (63f27a8) with main (98f68e1)}

:tada: Hooray! pytest-codspeed just leveled up to 3.0.0!

A heads-up, this is a breaking change and it might affect your current performance baseline a bit. But here's the exciting part - it's packed with new, cool features and promises improved result stability :partying_face:! Curious about what's new? Visit our releases page to delve into all the awesome details about this new version.

Summary

✅ 11 untouched benchmarks
🆕 2 new benchmarks

Benchmarks breakdown

	Benchmark	main	feat/frac-diff	Change
🆕	test__frac_diff__benchmark_polars[trade_data0]	N/A	1.4 ms	N/A
🆕	test__get_weights_ffd__benchmark_rs	N/A	258.8 µs	N/A