In Duckietown_graph_builder, added a Node class with its subclass MovableNode
Namings : node_type (eg "duckiebot") is almost no longer used, node_id is the full name (eg. "duckiebot_01") and node_number is the number (eg "01"). The node class make that each node is always represented only by its node_id, or by the object node itself in the DuckietownGraphBuilder class
Added also seperate locks for each class, that will protect their internal params to be accessed multiple times at once.
Main changes: