Only sort in CoverTree::find function if there are more than k points

Sorting only needs to be done if the list of KNN candidates is greater
than length k.

src/algorithm/neighbour/cover_tree.rs

@@ -180,7 +180,9 @@ impl<T: Debug + PartialEq, F: RealNumber, D: Distance<T, F>> CoverTree<T, F, D>
             }
         }
         
-        neighbors.sort_by(|a, b| a.1.partial_cmp(&b.1).unwrap());
+        if neighbors.len() > k {
+            neighbors.sort_by(|a, b| a.1.partial_cmp(&b.1).unwrap());
+        }
         Ok(neighbors.into_iter().take(k).collect())
     }