Finalize folds implementation

[klwetstone]

Final pieces of #41 (training with folds)

• Update evaluation code to get feature importances
• If someone doesn't have region and n_folds > 1, warn when doing model training and train without folds. Change _prep_train_data so that we don't error if region is not provided and n_folds > 1
• Also warn if someone has n_folds > 1 and fewer samples than n_folds
• change if / else order in training so that first we check whether we're training with multiple folds