Feat/operator squeeze unsqueeze

Context

see issue : aidge_core#65 (closed)

✔ Refactored tests for onnx

• Test_import_export can now easily run multiple networks
• node register has been completed

✔ replaced all prints by logging

✔ Fixed onnx node opset retrieval

Done in 190c2d9f

Previously the opset of the model was retrieved with model.ir_version, which is .... I do not know what it is but this is named intermediate representation and not opset.

Opsets have to be retrieved via model.opset_import. Its a list [opset] (/protoobject) that contains :

1. a domain name
2. an associated opset

Each node is attached to a domain name which bind them to a given opset.

This allos to use multiple opsets in a given model.

More on that here : http://onnx.ai/sklearn-onnx/auto_tutorial/plot_cbegin_opset.html