twacube: atomic propositions are now passed by copy

Passing atomic propositions by reference allows to
save very little memory so it doesn't worth complexifying
memory management.

* spot/twacube/twacube.cc, spot/twacube/twacube.hh: here.

spot/twacube/twacube.cc

 // -*- coding: utf-8 -*-
-// Copyright (C) 2015 Laboratoire de Recherche et Developpement de
+// Copyright (C) 2015, 2016 Laboratoire de Recherche et Developpement de
 // l'EPITA (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -52,7 +52,7 @@ namespace spot
   transition::transition()
   { }
 
-  twacube::twacube(const std::vector<std::string>& aps):
+  twacube::twacube(const std::vector<std::string> aps):
     init_(0U), aps_(aps), cubeset_(aps.size())
   {
   }
@@ -74,7 +74,7 @@ namespace spot
     return acc_;
   }
 
-  const std::vector<std::string>&  twacube::get_ap()
+  std::vector<std::string> twacube::get_ap()
   {
     return aps_;
   }

spot/twacube/twacube.hh

@@ -105,11 +105,11 @@ namespace spot
   {
   public:
     twacube() = delete;
-    twacube(const std::vector<std::string>& aps);
+    twacube(const std::vector<std::string> aps);
     virtual ~twacube();
     const acc_cond& acc() const;
     acc_cond& acc();
-    const std::vector<std::string>& get_ap();
+    std::vector<std::string> get_ap();
     unsigned int new_state();
     void set_initial(unsigned int init);
     unsigned int get_initial();
@@ -146,7 +146,7 @@ namespace spot
   private:
     unsigned int init_;
     acc_cond acc_;
-    const std::vector<std::string>& aps_;
+    const std::vector<std::string> aps_;
     graph_t theg_;
     cubeset cubeset_;
   };