spatial/vptree: return error rather than panic

spatial/vptree/vptree_simple_example_test.go

@@ -6,6 +6,7 @@ package vptree_test
 
 import (
 	"fmt"
+	"log"
 
 	"gonum.org/v1/gonum/spatial/vptree"
 )
@@ -21,7 +22,10 @@ func ExampleTree() {
 		vptree.Point{7, 2},
 	}
 
-	t := vptree.New(points, 3, nil)
+	t, err := vptree.New(points, 3, nil)
+	if err != nil {
+		log.Fatal(err)
+	}
 	q := vptree.Point{8, 7}
 	p, d := t.Nearest(q)
 	fmt.Printf("%v is closest point to %v, d=%f\n", p, q, d)
@@ -41,7 +45,10 @@ func ExampleTree_Do() {
 	}
 
 	// Print all points in the data set within 3 of (3, 5).
-	t := vptree.New(points, 0, nil)
+	t, err := vptree.New(points, 0, nil)
+	if err != nil {
+		log.Fatal(err)
+	}
 	q := vptree.Point{3, 5}
 	t.Do(func(c vptree.Comparable, _ int) (done bool) {
 		// Compare each distance and output points