Function initial_guess 

fn initial_guess(
    y: &[f64],
    inside: &[bool],
    dk: &[f64],
    support: &Box3,
    ws: &mut Fft3dWorkspace,
) -> Option<Vec<Complex64>>

Step 3: x₀ = z / max|z| with z = M F⁻¹(K F(M F⁻¹(K F(M F⁻¹(D⁺ F y)))))). y is zero outside inside, which lies in support. None if z is zero.