Slightly less wasteful data copy

handouts/src/24-resourceful.md

@@ -829,9 +829,11 @@ impl Concentrations {
 
         // Access the CPU-side buffer
         let v_data = self.v_buffer.read()?;
-        for (buf, elem) in v_data.iter().zip(self.v_ndarray.iter_mut()) {
-            *elem = *buf;
-        }
+        let v_target = self
+            .v_ndarray
+            .as_slice_mut()
+            .expect("Failed to access ndarray as slice");
+        v_target.copy_from_slice(&v_data);
         Ok(&self.v_ndarray)
     }