twacube: mark get_ap method as const

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

spot/twacube/twacube.cc

@@ -66,7 +66,7 @@ namespace spot
     return acc_;
   }
 
-  std::vector<std::string> twacube::get_ap()
+  std::vector<std::string> twacube::get_ap() const
   {
     return aps_;
   }

spot/twacube/twacube.hh

@@ -139,7 +139,7 @@ namespace spot
     acc_cond& acc();
 
     /// \brief Returns the names of the atomic properties.
-    std::vector<std::string> get_ap();
+    std::vector<std::string> get_ap() const;
 
     /// \brief This method creates a new state.
     unsigned new_state();