graph: rename nodeFilterIterator => nodeIteratorPair and minor fixes

The length calculation is now correct. This had not effect previously as
NodesOf is lenient in how it handles mis-estimated iterator lengths.

graph/export_test.go

@@ -0,0 +1,9 @@
+// Copyright ©2019 The Gonum Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package graph
+
+var (
+	NewNodeIteratorPair = newNodeIteratorPair
+)