Add a unique node_id attributes in the node.

Context

In the context of creating a unique id to identify nodes for the model explorer plugin, I propose to add an attribute to every node: node_id this attribute is set thanks to a global.

For context here is the MR of model explorer: aidge_model_explorer!3 (merged)

Modified files

• Node.hpp: Add a static counter that is incremented when creating a new Node
• Node.cpp: Update ctor to set node_id and increment NodeID global var