Function helium_electron_fraction

pub fn helium_electron_fraction(z: f64, cosmo: &Cosmology) -> f64

Helium electron contribution: free electrons per H atom from He.

x_He = f_He × (y_HeI + 2 × y_HeII) where y_HeII is the doubly-ionized fraction and y_HeI is the singly-ionized fraction.