Function wave_vector_z 

pub fn wave_vector_z(kx: Complex, ky: Complex, k: Complex) -> Complex

The normal wavenumber kz = sqrt(k^2 - kx^2 - ky^2) on the root with a nonnegative imaginary part, so the wave propagates or decays toward +z; zero at a cutoff.

Upstream: treams.misc.wave_vec_z.