pub fn compact_second_derivative(u: &[f64], dx: f64) -> Vec<f64>
Compact (Padé) 4th-order second derivative.
Solves (1/10) f''[i-1] + f''[i] + (1/10) f''[i+1] = (12/10)(u[i+1]-2u[i]+u[i-1])/dx².
(1/10) f''[i-1] + f''[i] + (1/10) f''[i+1] = (12/10)(u[i+1]-2u[i]+u[i-1])/dx²