Cleanups

[rcurtin]

These aren't really changes to functionality but more changes to organization to bring this into line with the rest of the mlpack code. I made these changes to what used to be the model_parser/ directories:

• Fix style to match mlpack style (2-space tabs, a few other things)
• Use size_t instead of int where possible.
• Don't do using namespace in a header file.
• Move to onnx_mlpack/ directory (so that all inclusions eventually will be under this directory).
• Remove onnx_pb.h and use the one provided by the system.

I think that the conversions to/from mlpack/ONNX weight formats can be simplified a lot, but I'll do that in the future.