config validation

[aryarm]

There are a lot of config options and it might be hard for a user to know how to fill them, despite the comments in the config file. It would be great if we could validate the config file that the user provides.

Snakemake lets us use JSON schemas to do this, but I think we'll need something a lot more robust, especially if JSON schemas don't allow us to conditionally require options based on other options.

Perhaps the best way to proceed would be to create a validation python module that uses argparse or something similar? We could import that module in the Snakefiles.