fn del2(f: &[f64], (nx, ny, nz): (usize, usize, usize), h: [f64; 3]) -> Vec<f64>Expand description
MATLAB’s del2(f, hx, hy, hz) with the spacings already in array-axis order (h[0] along
the first axis): per axis, half the centred second difference inside and linear
extrapolation at the ends, summed over the axes and divided by 3. Coordinates are h·(1:n)
and their differences are taken as MATLAB takes them.