separate metadata and trace data

[jonorthwash]

Currently metadata and traces are both stored in metadata.json.

This can be problematic, because sometimes the metadata needs to be refreshed (e.g., when working on data in a new directory, because of #156), which can currently only be done manually or by deleting the file (#152).

It would be nice to have metadata.json for everything but traces, and traces.json for traces. At least one of the files should track the other–probably at least the former should have the filename of the latter, maybe replacing a traces datatype with a traces_filename field, for most robust compatibility.

[Quaratz13]

Overview:

• [ ] manually separate files (metadata and traces file) (include traces file in metadata)
• [ ] write code to load the new traces file
• [ ] write code to check if old or new type and decide which method to use to load it
• [ ] write code to separate the file types (if it's all in the old metadata file)
• [ ] prompt user if they want the files separated or use the old type)