Removing references to jets and various other improvements

This MR contains several quality of life improvements for salt

main changes:

• references to the string "jets" are now replaced with a variable global_object, which is set to "jets" by default
• the InitNet class now handles input concatenation, rather than the dataloader. This simplifies ONNX export.
• simplified normalisation now happens in a dedicated InputNorm class. this is because it's easier to normalise before concatenation.
• fix onnx export by closing #38 (closed) (also fix a bug introduced by !157 (merged))

configuration:

• you can now use directly the name of the h5 dataset in configs (e.g. for the variables block and input_name keys for InitNet and task classes). there is still a data.input_map option to map these to new values externally if you get an h5 with different dataset names, this mapping defaults to the identity mapping if not specified.
• "special" variable keys are now capitalised, e.g. EDGE, GLOBAL and PARAMETERS
• InitNet and task classes now have a standardised input_name key, rather than mixing input_type and input_name
• the gnn block is renamed to encoder which is more general
• num_jets_train etc are renamed to num_train etc

other:

• common type alias are defined in salt.utils.typing.
• LightningTagger is renamed to SaltModelWrapper
• various documentation improvements