Statistical assumptions module

Shapiro-Wilk implementation adapted from SciPy swilk (Royston AS R94),
restricted to uncensored samples. Normal inverse and polynomial coefficients
follow SciPy; normal tails use AS66 and a continued fraction for log tails.
Levene's F probability uses jStat (MIT).
Anderson-Darling p-value approximation adapted from statsmodels normal_ad.

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