Example to export model to ONNX

[twoertwein]

Closes #689

I added an example to demonstrate that it is possible to convert an RSMTool model to ONNX.

I just implemented the transformations to make the conversion work for the model I was dealing with :) The example will error when it encounters skll/rsmtool features that it does not (yet) support/when the conversion fails.

I was dealing with a simple regression model. fast_predict takes 0.1s while the converted ONNX+numpy model takes only 0.00014s