docs: public static void write(ClassificationDataSet data, OutputStream os)
      {
          PrintWriter writer = new PrintWriter(os);
          for(int i = 0; i < data.size(); i++)
          {
              int pred = data.getDataPointCategory(i);
              Vec vals = data.getDataPoint(i).getNumericalValues();
              writer.write(pred + " ");
              for(IndexValue iv : vals)
              {
                  double val = iv.getValue();
                  if(Math.rint(val) == val)//cast to long before writting to save space
                      writer.write((iv.getIndex()+1) + ":" + (long)val + " ");//+1 b/c 1 based indexing
                  else
                      writer.write((iv.getIndex()+1) + ":" + val + " ");//+1 b/c 1 based indexing
              }
              writer.write("\n");
          }
          writer.flush();
          writer.close();
      }