spatial/r3: harmonize Vec API with num/quat.Number

Migration to the new API can be achieved with this rsc.io/rf script:

```
rf ex {
	import "gonum.org/v1/gonum/spatial/r3";
	var p,q r3.Vec;
	var f float64;

	p.Add(q) -> r3.Add(p, q);
	p.Sub(q) -> r3.Sub(p, q);
	p.Scale(f) -> r3.Scale(f, p);
	p.Dot(q) -> r3.Dot(p, q);
	p.Cross(q) -> r3.Cross(p, q);
	p.Rotate(f, q) -> r3.Rotate(p, f, q);
}
```

Updates gonum/gonum#1553.

spatial/r3/deprecated.go

@@ -0,0 +1,49 @@
+// Copyright ©2021 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 r3
+
+// TODO(sbinet): remove this file for Gonum-v0.10.0.
+
+// Add returns the vector sum of p and q.
+//
+// DEPRECATED: use r3.Add.
+func (p Vec) Add(q Vec) Vec {
+	return Add(p, q)
+}
+
+// Sub returns the vector sum of p and -q.
+//
+// DEPRECATED: use r3.Sub.
+func (p Vec) Sub(q Vec) Vec {
+	return Sub(p, q)
+}
+
+// Scale returns the vector p scaled by f.
+//
+// DEPRECATED: use r3.Scale.
+func (p Vec) Scale(f float64) Vec {
+	return Scale(f, p)
+}
+
+// Dot returns the dot product p·q.
+//
+// DEPRECATED: use r3.Dot.
+func (p Vec) Dot(q Vec) float64 {
+	return Dot(p, q)
+}
+
+// Cross returns the cross product p×q.
+//
+// DEPRECATED: use r3.Cross.
+func (p Vec) Cross(q Vec) Vec {
+	return Cross(p, q)
+}
+
+// Rotate returns a new vector, rotated by alpha around the provided axis.
+//
+// DEPRECATED: use r3.Rotate
+func (p Vec) Rotate(alpha float64, axis Vec) Vec {
+	return Rotate(p, alpha, axis)
+}