message: Update DataFrame.h
  Corrected a comment
diff: @@ -82,8 +82,10 @@ using DataFrameView = DataFrame<TS, HeteroView>;
  // ----------------------------------------------------------------------------
  -// DS is a data storage container. It must have an interface partly identical
  -// to std::vector
  +// TS: Index(e.g. Timestamp) type. Although an index column need not necessarily
  +// represent time.
  +// HETERO: See the static assert below. It can only be either
  +// a HeteroVector (StdDataFrame) or a HeteroView (DataFrameView)
  //
  template<typename TS, typename HETERO>
  class DataFrame {
  