Improve crossover function

Rather than always choosing a parental weight verbatim, weights are
sometimes averaged between parents.

src/nn.rs

@@ -68,10 +68,16 @@ impl NN {
                 .iter()
                 .zip(b.weights.iter())
                 .map(|(m1, m2)| {
-                    m1.zip_map(
+                    m1.zip_map(m2, |ele1, ele2| {
-                        m2,
+                        let choice = gen_range(0., 3.);
-                        |ele1, ele2| if gen_range(0., 1.) < 0.5 { ele1 } else { ele2 },
+                        if choice < 1. {
-                    )
+                            ele1
+                        } else if choice < 2. {
+                            ele2
+                        } else {
+                            (ele1 + ele2) / 2.
+                        }
+                    })
                 })
                 .collect(),
         }