Function pressure_force_kernel 

pub fn pressure_force_kernel(
    positions: &[[f64; 3]],
    densities: &[f64],
    pressures: &[f64],
    masses: &[f64],
    h: f64,
    neighbor_list: &NeighborList,
) -> Vec<[f64; 3]>

Symmetric SPH pressure force kernel using neighbour lists.

F_i^press = -m_i sum_j m_j (p_i/rho_i^2 + p_j/rho_j^2) nabla W(r_ij, h)