code: public ValRow slice(int[] cols) {
      double[] ds = new double[cols.length];
      String[] ns = new String[cols.length];
      for (int i = 0; i < cols.length; ++i) {
        ds[i] = _ds[cols[i]];
        ns[i] = _names[cols[i]];
      }
      return new ValRow(ds, ns);
    }
docs: Creates a new ValRow by selecting elements at the specified indices.
  @param cols array of indices to select. We do not check for AIOOB errors.
  @return new ValRow object