[FEAT] ARIMA analytical gradient

[elephaint]

Adds analytical gradient for ARIMA (rather then relying on internal minimize estimates from scipy's BFGS).

Speeds up about 2-5x, see the benchmark script.

[codspeed-hq]

Merging this PR will not alter performance

✅ 6 untouched benchmarks

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_{Comparing feat/improve_arima (eaa9962) with main (65569e1)}