Feat/operator global average pooling

Context

• Adding support for GlobalAveragePooling Operator (needed to implement EfficientNet and MobileNet-v2). No linked gitlab issue.

• Contains a fix for GraphView::remove that impaired removeFlatten execution, I added tests for remove flatten & Scheduler at the same time. I refactored a bit GraphView::replace to make the code a bit more readable (more explicit var name and types).

• Added an friend ostream operator<< () for node (commented)

Linked MR:

• aidge_backend_cpu!42 (merged)
• aidge_onnx!30 (merged)