message: commented out visit_asyncs
diff: @@ -1499,6 +1499,7 @@ 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);
  @@ -1506,8 +1507,7 @@ public: // Visitors
  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
  