docs: public double max_per_class_error() {
      int n = nclasses();
      if(n == 0)throw new UnsupportedOperationException("max per class error is only defined for classification problems");
      double res = classErr(0);
      for(int i = 1; i < n; ++i)
        res = Math.max(res,classErr(i));
      return res;
    }