Remove explicit references to jets and generalise global object classification

I was recently "borrowing" some salt code and needed to remove a few references to jets. I think now the only things that need updating (outside of things that do specifically relate to jets and we won't update) is the h5 file attributes for number of training jets which needs updating in UPP and the ONNX conversion script which can be left to a later MR.

Closes #46 (closed)