add option to only save top_n_features by feature importance, to save db size

[ClimbsRocks]

really, we should only have to save the top 50-100 features, max, to get a good sense for feature discrepancies.

[ClimbsRocks]

OR, let the user specify which features to save. that way if there are metadata fields they want to use for analytics or recording purposes, they can, while still optimizing db size.