pub fn stencil3(x: &[f32], y: &mut [f32])
Stencil: 1-D 3-point Laplacian y[i] = x[i-1] - 2·x[i] + x[i+1], with the ends clamped (one-sided zero). y.len()==x.len().
Stencil
y[i] = x[i-1] - 2·x[i] + x[i+1]
y.len()==x.len()