graph: revert NewNodeID -> NewNode

Created by: kortschak

@mewmew @vladimir-ch @btracey Please take a look.

This is a minimal change. The rationale is explained in the issue, but briefly, by returning a graph.Node rather than an int64, the destination graph can decide what type of node is added, where previously this was hard coded into the filling function. A good example is in the duplication.go file which generates random duplication graphs.

Fixes #59 (closed).