          optional subset of the training set. The indices in <code>subsampleIdx</code>
                                        are counted relatively to the <code>_sample_idx</code>,
                                        passed to the
                                        <code>CvDTreeTrainData</code> constructor. For example, if
                                        <code>_sample_idx=[1,
                                            5, 7, 100]</code>, then <code>subsampleIdx=[0,3]</code>
             