Function shallow_water::stafft::forrdx6

pub fn forrdx6(
    a: ArrayView3<'_, f64>, 
    b: ArrayViewMut3<'_, f64>, 
    nv: usize, 
    lv: usize, 
    cosine: ArrayView2<'_, f64>, 
    sine: ArrayView2<'_, f64>
)

Radix six physical to Hermitian FFT with 'decimation in time'.