unit: Update documentation, create examples, and comment types

Updates 90.

unit/unitexample_test.go

@@ -0,0 +1,29 @@
+// Copyright ©2017 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 unit_test
+
+import (
+	"fmt"
+
+	"gonum.org/v1/gonum/unit"
+)
+
+func ExampleNew() {
+	// Create an acceleration of 3 m/s^2
+	accel := unit.New(3.0, unit.Dimensions{unit.LengthDim: 1, unit.TimeDim: -2})
+	fmt.Println(accel)
+
+	// Output: 3 m s^-2
+}
+
+func ExampleNewDimension() {
+	// Create a "trees" dimension
+	// Typically, this should be used within an init function
+	treeDim := unit.NewDimension("tree")
+	countPerArea := unit.New(0.1, unit.Dimensions{treeDim: 1, unit.LengthDim: -2})
+	fmt.Println(countPerArea)
+
+	// Output: 0.1 tree m^-2
+}