message: use f64 to determine the duration time
diff: +#![feature(duration_float)]
  +
  extern crate rayon;
  mod laplace;
  @@ -138,7 +140,7 @@ fn laplace(size_x: usize, size_y: usize) -> Result<()> {
  let (iterations, res) = laplace::compute(matrix, size_x, size_y);
  println!(
  "Time to solve {} s, iterations {}, residuum {}",
  - now.elapsed().as_secs(),
  + now.elapsed().as_secs_f64(),
  iterations,
  res
  );
  