pub fn cdf(t: f64, nu: f64) -> f64
cdf P(T ≤ t) with nu degrees of freedom. Uses I_x(ν/2, ½) with x = ν/(ν+t²) and the sign of t.
P(T ≤ t)
nu
I_x(ν/2, ½)
x = ν/(ν+t²)
t