chore: fix clippy warnings in NSGA-III, SPEA2, and compare example

- nsga3: drop redundant `.into_iter()` in extend call; use
  `#[allow(clippy::needless_range_loop)]` on the back-substitution
  loop where `j` indexes into the matrix; remove an unneeded
  `return` keyword in a closure.
- spea2: switch `pool.extend(x.drain(..))` to `pool.append(&mut x)`.
- examples/compare.rs DTLZ2 evaluator: same `needless_range_loop`
  silencer on the inner cosine product loop.

examples/compare.rs

@@ -79,6 +79,7 @@ impl Problem for Dtlz2 {
         let mut f = vec![0.0_f64; m];
         for i in 0..m {
             let mut prod = scale;
+            #[allow(clippy::needless_range_loop)] // Body indexes `x[j]`.
             for j in 0..(m - i - 1) {
                 prod *= (x[j] * std::f64::consts::FRAC_PI_2).cos();
             }