Allow KNN with k=1

src/algorithm/sort/heap_select.rs

@@ -41,6 +41,9 @@ impl<'a, T: PartialOrd + Debug> HeapSelection<T> {
 
     pub fn heapify(&mut self) {
         let n = self.heap.len();
+        if n <= 1 {
+            return;
+        }
         for i in (0..=(n / 2 - 1)).rev() {
             self.sift_down(i, n - 1);
         }

src/neighbors/knn_regressor.rs

@@ -116,9 +116,9 @@ impl<T: RealNumber, D: Distance<Vec<T>, T>> KNNRegressor<T, D> {
             )));
         }
 
-        if parameters.k <= 1 {
+        if parameters.k < 1 {
             return Err(Failed::fit(&format!(
-                "k should be > 1, k=[{}]",
+                "k should be > 0, k=[{}]",
                 parameters.k
             )));
         }