initial ONNX support

[Hoeze]

The prediction results do not match yet, see interpretml/ebm2onnx#8

[Hoeze]

@WagnerNils Can you merge this please?
This converts your models to ONNX and removes the need to install any interpret package for pure inference.

Fun fact:
Did you know that "splice_site_is_expressed" in your EBM gets converted from int -> float -> string -> int?
E.g. splice_site_is_expressed == 1 (int) -> 1.0 (float) -> "1.0" (string) -> 0 (int) 🤦
I manually fixed this in the onnx models using onnx-modifier.