Add header to meta_model

proofs/meta_model.why

+(**************************************************************************)
+(*                                                                        *)
+(*  This file is part of the Frama-C's MetACSL plug-in.                   *)
+(*                                                                        *)
+(*  Copyright (C) 2018-2022                                               *)
+(*    CEA (Commissariat à l'énergie atomique et aux énergies              *)
+(*         alternatives)                                                  *)
+(*                                                                        *)
+(*  you can redistribute it and/or modify it under the terms of the GNU   *)
+(*  Lesser General Public License as published by the Free Software       *)
+(*  Foundation, version 2.1.                                              *)
+(*                                                                        *)
+(*  It is distributed in the hope that it will be useful,                 *)
+(*  but WITHOUT ANY WARRANTY; without even the implied warranty of        *)
+(*  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the         *)
+(*  GNU Lesser General Public License for more details.                   *)
+(*                                                                        *)
+(*  See the GNU Lesser General Public License version 2.1                 *)
+(*  for more details (enclosed in the file LICENSE)                       *)
+(*                                                                        *)
+(**************************************************************************)
+
 module Meta_Base
     use set.Fset as S
     use fmap.Fmap as M