title: Retrieve list of training features names from classifier
body: <p>Is there a way to retrieve the list of feature names used for training of a classifier, once it has been trained with the <code>fit</code> method? I would like to get this information before applying to unseen data.
  The data used for training is a pandas <code>DataFrame</code> and in my case, the classifier is a <code>RandomForestClassifier</code>.</p>