"This function returns three outputs: a list of training feature vectors, each containing zero crossing rate and spectral centroid; the ground truth labels of the training data; and a `scaler` object of type [`sklearn.preprocessing.MinMaxScaler`](http://scikit-learn.org/stable/modules/generated/sklearn.preprocessing.MinMaxScaler.html). `scaler` simply scales every training feature vector such that the two features lie between -1 and 1."