message: Update DataFrame.h
  Added more comments
diff: @@ -89,9 +89,10 @@ using DataFrameView = DataFrame<TS, HeteroView>;
  // ----------------------------------------------------------------------------
  // TS: Index(e.g. Timestamp) type. Although an index column need not necessarily
  -// represent time.
  +// represent time. Basically TS could be any built-in or user-defined type.
  // HETERO: See the static assert below. It can only be either
  // a HeteroVector (StdDataFrame) or a HeteroView (DataFrameView)
  +// A DataFrame can contain columns of any built-in or user-defined types.
  //
  template<typename TS, typename HETERO>
  class DataFrame {
  