tk/Dataframe: added columns

lib/tk/dataframe.ml

@@ -16,6 +16,8 @@ type t = {
   cols : (string * column) list ;
 }
 
+let columns df = df.cols
+
 let nrows t = t.nrows
 let ncols t = t.ncols
 

lib/tk/dataframe.mli

@@ -15,6 +15,8 @@ val get_col : t -> int -> column option
 
 val get_col_by_name : t -> string -> column option
 
+val columns : t -> (string * column) list
+
 val from_file :
   ?header:[`Read_in_file | `Expect of string list | `Use of string list] ->
   string ->