Function parallel_generate_sparse_density_map 

pub fn parallel_generate_sparse_density_map<I: Index, R: Real>(
    grid: &UniformGrid<I, R>,
    particle_positions: &[Vector3<R>],
    particle_densities: &[R],
    active_particles: Option<&[usize]>,
    particle_rest_mass: R,
    compact_support_radius: R,
    cube_size: R,
) -> Result<OwnedDensityMap<I, R>, DensityMapError<R>>

Computes a sparse density map for the fluid based on the specified background grid, multi-threaded implementation