Function kernel_gradient 

pub fn kernel_gradient(
    r_vec: [f64; 3],
    r: f64,
    params: &SphKernelParams,
    kernel: SphKernel,
) -> [f64; 3]

Evaluate the gradient nabla W(r_vec, h) of the smoothing kernel.

r_vec is the vector from particle j to particle i (r_i - r_j). r is its magnitude. Returns the zero vector when r < 1e-12 or r >= h.