Generic class to handle Graphdb instances (+ vanilla models)

[pdonorio]

The final protocol should be (inside neo4j.py):

• Init of the graph at flask server startup. To check if the graph is reachable.
• Destroy the instance from above
• Import from your code the GraphFarmer class and extend it
• Use self.get_graph_instance(models)

[pdonorio]

Discussed with @mdantonio and solved