Adding Zeta to the top level with tests

zeta.go

@@ -0,0 +1,12 @@
+// Copyright ©2016 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 mathext
+
+import "github.com/zeroviscosity/mathext/internal/cephes"
+
+// Zeta computes the Riemann zeta function of two arguments
+func Zeta(x, q float64) float64 {
+	return cephes.Zeta(x, q)
+}