 This is only used for preprocess_mode='standard'.
    maxlen (int):  maximum length of tokens in document
    ngram_range (int):  If &gt; 1, will include 2=bigrams, 3=trigrams and bigrams
    train_test_names (list):  list of strings represnting the subfolder
                             name for train and validation sets
                             if test name is missing, &lt;val_pct&gt; of training
                             will be used for validation
    preprocess_mode (str):  Either 'standard' (n