Function penetrability 

pub fn penetrability(l: u32, rho: f64) -> f64

Penetrability factor P_l(ρ) for orbital angular momentum l.

Returns the ratio of the outgoing wave amplitude at the nuclear surface to the asymptotic amplitude. For s-wave (l=0), P = ρ.

Reference: SAMMY rml/mrml07.f Pf function (lines 474-522)