Support opset version 9

[WilliamTambellini]

As today, if trying to import an onnx file using opset 9 :

$ example/docclass_example
Document classifier example
terminate called after throwing an instance of 'menoh::error'
what(): menoh unsupported onnx opset version error: given onnx has opset version 9 > 8
Aborted

opset 9 chnagelog :
https://github.com/onnx/onnx/blob/master/docs/Changelog.md#version-9-of-the-default-onnx-operator-set