message: Added the method signatures for filters. Implementaion to follow
diff: @@ -638,6 +638,40 @@ public: // Read/access interfaces
  //
  inline TSVec &get_index () { return (indices_); }
  + // [Not Implemented Yet]
  + // It returns a DataFrame based on the filters passed. Each value in each
  + // column specified in args is passed to the correpsonding filter functor.
  + // If filter returns true, the value is included, otherwise the value is
  + // excluded.
  + // All filter functors must have the following signature:
  + // bool (const TimeStamp &i, const char *name, const T &col_value)
  + //
  + // types: The list of types for columns in args
  + // args: A variable list of arguments consisting of
  + // std::pair(<const char *name,
  + // &std::function<bool (const TimeStamp &, const char *, const T &)>).
  + // Each pair represents a column name and the functor to filter it.
  + //
  + template<typename ... types>
  + DataFrame get_data_by_filter (types ... args) const;
  +
  + // [Not Implemented Yet]
  + // It behaves like get_data_by_filter(), but it returns a DataFrameView.
  + // A view is a DataFrame that is a reference to the original DataFrame.
  + // So if you modify anything in the view the original DataFrame will
  + // also be modified.
  + // Note: There are certain operations that you cannot do with a view.
  + // For example, you cannot add/delete columns, etc.
  + //
  + // types: The list of types for columns in args
  + // args: A variable list of arguments consisting of
  + // std::pair(<const char *name,
  + // &std::function<bool (const TimeStamp &, const char *, const T &)>).
  + // Each pair represents a column name and the functor to filter it.
  + //
  + template<typename ... types>
  + DataFrameView<TS> get_view_by_filter (types ... args) const;
  +
  // This is the most generalized visit function. It visits multiple
  // columns with the corresponding function objects sequentially.
  // Each function object is passed every single value of the given
  @@ -651,7 +685,9 @@ public: // Read/access interfaces
  // Ts: The list of types for columns in args
  // args: A variable list of arguments consisting of
  // std::pair(<const char *name,
  - // &std::function<bool (const TimeStamp &, const char *, [const] T &)>).
  + // &std::function<bool (const TimeStamp &,
  + // const char *,
  + // [const] T &)>).
  // Each pair represents a column name and the functor to run on it.
  // NOTE: The second member of pair is a _pointer_ to the function or
  // functor object
  