Function apply_dipole 

fn apply_dipole(
    fft_ws: &mut Fft3dWorkspace,
    k: &[f64],
    x: &[f64],
    cbuf: &mut [Complex64],
    out: &mut [f64],
)

Forward dipole convolution: Dx = real(ifft(kernel .* fft(x))).

cbuf is scratch of length n_total; result written to out.