graph/graphs/gen: use graph construction interfaces correctly

Created by: kortschak

This makes the simple graph generators use the NewNode and NewEdge methods rather than always using simple.Node and simple.Edge. It also makes the (sub)graphs that are generated actually reflect the theoretical statistics. This means that they no longer add edges to already existing nodes. We may want to add in functions that do do that at some stage (or change the behaviour of what I have here to allow that).

Please take a look.

Fixes #602 (closed).