Cleanup for flavor tagging GNN tool

This adds a few features to the code surrounding the flavor tagging GNN:

• Split the core functionality into a tool and a simple C++ class.
• Make both DL2HighLevel and GNN copy and move construable.
• Clean up some unnecessary class member data.
• Allow the user to omit the second argument of get-onnx-metadata (the metadata key). Without the key, prints a list of possible keys.
• Disable all non-fatal logging for the GNN: many of the warnings are harmless and are useless in a production environment.

We need this for two reasons:

• To keep the code manageable. I've had to hack around the first two issues.
• Do ensure that debugging the GNN tagger is as painless as possible.

It should change no outputs.

Tagging @thuffman, @duperrin.