Function satterthwaite_sf 

pub fn satterthwaite_sf(q: f64, weights: &[f64]) -> f64

Satterthwaite two-moment approximation to P(Σ λ_i χ²₁ > q): match mean and variance with g · χ²_ν, g = Σλ²/Σλ, ν = (Σλ)²/Σλ².