spatial/r1: import and rename from bound

* spatial/r1: import and rename from bound

Mechanically generated with:

 $> gomvpkg -from gonum.org/v1/gonum/bound -to gonum.org/v1/gonum/spatial/r1 -vcs_mv_cmd "git mv {{.Src}} {{.Dst}}"

Fixes #956.

spatial/r1/interval.go

@@ -0,0 +1,10 @@
+// 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 r1
+
+// Interval represents an interval.
+type Interval struct {
+	Min, Max float64
+}