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);

examples/kickscore-basics.rs

@@ -21,9 +21,9 @@ fn main() {
     ];
 
     // Now we are ready to add the items in the model.
-    model.add_item("Spike", Box::new(k_spike));
-    model.add_item("Tom", Box::new(k_tom));
-    model.add_item("Jerry", Box::new(k_jerry));
+    model.add_item("Spike", k_spike);
+    model.add_item("Tom", k_tom);
+    model.add_item("Jerry", k_jerry);
 
     // At first, Jerry beats Tom a couple of times.
     model.observe(&["Jerry"], &["Tom"], 0.0);

examples/nba-history.rs

@@ -62,7 +62,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
             Box::new(ks::kernel::Matern32::new(0.138, 1.753 * seconds_in_year)),
         ];
 
-        model.add_item(&team, Box::new(kernel));
+        model.add_item(&team, kernel);
     }
 
     for (winner, loser, t) in observations {