Graph Entry and exit Nodes

[jonassorgenfrei]

I think it would be cool to add 2 functions to the Graph implementation.

To get a list with all of the EntryPointNodes to the graph that are the first Nodes in the graph execution (dont have any input connection to other nodes)
To get a list with all of the ExitPointNodes of the graph that are the last Nodes of the graph execution (dont have any output connection to other nodes)

So it's possible to append or prepend nodes to a graph without knowing the structure of it.
esp. in combination with: #166