update Userguide and design doc for Node

Questions:

• How does "removeView" Node from the .hpp works? Form what i've seen, it removes the GraphView from the Node, but not the Node from the GraphView. The assymetry of information seem problematic
• getNodeDelta: does it only takes the Delta layer or all the layers between the Delta and the Node?
• Why is Node's addParent private and not public? How do we add parents in our Node?
• parentIsBackEdge: give the def of back edge with a little drawing because i like drawings

Linked MRs

aidge_core!231 (If you can, suggest a possible fix)