Function maxwell_diffusivity 

pub fn maxwell_diffusivity(d1: f64, d2: f64, volume_fraction2: f64) -> f64

Effective diffusion coefficient for a two-phase mixture (Maxwell model).

For spherical inclusions of phase 2 in matrix phase 1: D_eff = D1 * (D2 + 2D1 - 2phi2*(D1-D2)) / (D2 + 2D1 + phi2(D1-D2))