When levels provided, add_best_levels silently ignores other arguments

[michaellevy]

From a quick check, it looks like n_levels and outcome are silently ignored, groups and id are required, and arguments passed to pivot are honored. The pivot thing is dangerous, and it would be nice to store everything in attrs so that you can just provide the two tables and levels in deployment. And then issue a warning if other arguments are passed/are not identical to what's in attrs.