message: fixed a type spec so it passes VSC++ compiler
diff: @@ -281,8 +281,7 @@ public: // Load/append/remove interfaces
  [](const typename DataFrame<I, H>::IndexType &t_1,
  const typename DataFrame<I, H>::IndexType &t) ->
  typename DataFrame<I, H>::size_type {
  - return (
  - static_cast<typename DataFrame<I, H>::size_type>(t - t_1));
  + return (static_cast<std::size_t>(t - t_1));
  });
  // It appends val to the end of the index column.
  