export models as onnx

[felixbur]

instead of pickle, as it's more portable and many ANN models can't be pickeld

[felixbur]

or, i guess, better to just add an onnx export function.
Because it seems impossible to convert a loaded onnx model back to sklearn or torch

[felixbur]

it seems for sklearn models this tool could simply be used
https://github.com/onnx/sklearn-onnx

[felixbur]

i work on it
#206