Function apply_k2 

fn apply_k2(
    buf: &mut [Complex64],
    kx: &[f64],
    ky: &[f64],
    kz: &[f64],
    invert: bool,
)

Multiply (or, with invert, divide) a spectrum by K = kx² + ky² + kz²; DC → 0 on divide.