Shortcuts for FGG creation

[davidweichiang]

When writing #99 I found myself frequently writing things like

g.add_edge(el, [node1, node2])

instead of

g.add_edge(Edge(el, [node1, node2]))

I wonder if it would make sense to provide some convenience functions to make FGG creation code more concise, or even to cut out some layers of data structure.

IMO the simplest way to create a graph would be

g = Graph()
node1 = g.add_node("foo")
node2 = g.add_node("bar")
g.add_edge("baz", [node1, node2])