message: Starting the async visit method implementations
diff: @@ -1499,6 +1499,16 @@ public: // Visitors
  return(const_cast<DataFrame *>(this)->visit<T, V>(name, visitor));
  }
  + template<typename T, typename V>
  + std::future<void>
  + visit_async(const char *name, V &visitor);
  +
  + template<typename T, typename V>
  + std::future<void>
  + visit_async(const char *name, V &visitor) const;
  +
  +
  +
  // It passes the values of each index and the two named columns to the
  // functor visitor sequentially from beginning to end
  //
  