Remove need to box kernel on model

examples/abcdef.rs

@@ -11,7 +11,7 @@ fn main() {
             Box::new(ks::kernel::Matern52::new(0.5, 1.0)),
         ];
 
-        model.add_item(player, Box::new(kernel));
+        model.add_item(player, kernel);
     }
 
     model.observe(&["A"], &["B"], 0.0);