NodeGraph IsValid #9

Determines if the node graph is... well.. valid.
The current approach is very naive and needs lots of work, basically return true if there exists an EndNode which you can reach from the start node.

Improvements:

1. Is there a path for which the EndNode is impossible to reach? (e.g in point 5)
2. Do all nodes have valid destination nodes?
3. Do all nodes have valid Choosers?
4. Are any exit nodes duplicates within a Node E.G a binary node (A)->(B,B) can just be a unary node (A)->(B).
5. Do any Unary nodes loop back to themselves? (A)->(A)
6. Is the start node an EndNode?