Adding Helper module

[stefkohub]

As agreed I have cleaned up the code of an Helper module, which is a partial port from Python ONNX library.
This patch is including three files:

• lib/axon_onnx/helper.ex which is the main helper module
• lib/axon_onnx/mapping.ex which is a module containing some constant conversion functions (e.g. from :FLOAT atom to its byte size)
• test/helper_module_test.exs which is containing some unit tests

[stefkohub]

I have some problem with mix format in CI/CD chain. It complains about deserialize.ex but I haven't touched it and this is the result of mix format on this file:

$ mix format --check-formatted lib/axon_onnx/deserialize.ex
$

[stefkohub]

Now it merged perfectly.