all: use fixdocs tool to store package documentation in doc.go

Add copyright header to doc.go

Keep original comment style (e.g. line comments and block comments).

Fix doc comments containing multiple line comments.

Remove vanity imports from all files except doc.go.

Fixes #12.

The source code of fixdocs is located at:
https://play.golang.org/p/7RtYLzldsO

graph/encoding/encoding.go

@@ -2,8 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package encoding provides a common graph encoding API.
-package encoding // import "gonum.org/v1/gonum/graph/encoding"
+package encoding
 
 import "gonum.org/v1/gonum/graph"