Very experimental add some way to get VNNLIB

[pobonomo]

This is kind a lot of things that needs testing the goal is to get as many vnn competition models understood.

Needs ONNX, new types of layer (conv2D, pooling,...), new network architectures (skip connections,...), and parsing vnnlib files

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[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

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