fixed index error with Eigen 3.3.7

[zhangbaozhe]

The original code cannot be compiled with Eigen 3.3.7 and raises the following error message

error: no match for ‘operator=’ (operand types are ‘Eigen::internal::enable_if<true, Eigen::IndexedView<Eigen::Matrix<double, -1, -1>, double, double> >::type’ {aka ‘Eigen::IndexedView<Eigen::Matrix<double, -1, -1>, double, double>’} and ‘double’)
  238 |               i * (i - 1) * (i - 2) * j * (j - 1) * (j - 2) * pow(ts, i + j - 5) / (i + j - 5);
      |                                                                                              ^

The index should be explicitly converted to size_t.

[w4c4dwsz]

That's right