Refactor Java ONNX Interface

[JackSullivan]

Description

A general refactoring of the tools used to produce ONNX models from Tribuo, together with a re-implementation of all existing Tribuo ONNX interfaces in terms of the refactored tool.

Motivation

The existing method of producing ONNX models for Tribuo requires manually wiring up the computation graph by passing magic strings into and out of ONNX operations, and requires an implementer to add each element individually to the constructed graph. Failing/forgetting to do each of these things for each node leads to invalid graphs that will potentially not be detected until much later.

This refactor provides a thin interface that doesn't require the user to manually track strings (while still allowing them to name constructed nodes as desired) ensures that references actually exist before they can be applied. It also allows programming interfaces in a more fluent style (input.apply(Operation, arguments)) and by confining direct interaction with ONNX protobufs to a small handful of methods opens the possibility for later enhancements to add much more in the way of early-failing correctness checking (like ensuring compatible dimensions between arguments).

[Craigacp]

This is a lot smaller and better checked, which is really nice. Most of the comments are asking for more docs.

I'm not sure if the ONNXRef subclasses should be static inner classes on ONNXRef. It would be nice to make them into records one day, but that means ONNXRef needs to be an interface.

[Craigacp]

LGTM