Add script accessor functions for nodes/edges of different types.

Fix and extend node/edge script accessor function of Graph data
structure plugin:
* Fix behavior of GraphStructure::list_nodes():
return nodes of all types
* Fix behavior of GraphStructure::list_edges():
return edges of all types
* Add new method GraphStructure::list_nodes(int type):
return only nodes of specified type
* Add new method GraphStructure::list_edges(int type):
return only edges of specified type