Declare/formalize a model file format

[NickleDave]

#672 describes replacing several options in the config file with a single option model_path.
The semantics of the model_path option are that it points to a directory generated by vak train. We should formalize this as a file format, ideally baked into the code, e.g. as an attrs class.

Since we don't have existing constraints we can put this in place now to make life easier later--I would rather capture as much info as possible, in a directory, about model provenance, training procedure, etc, instead of needing to put a lot of that into code (like the torchvision weights abstraction).

Currently the "file format" is:

• directory
• with log file
• config file
• checkpoints folder
  • that has "intermittent save" checkpoint as well as "best performance on validation set" checkpoint
• parts of transform pipeline, e.g. SpectScaler -- could we save the whole transform pipeline?
• for frame classification: labelmap.json