Make ExceptionAbstract::getStringMessage const

     * include/dynamic-graph/exception-abstract.h,
     * src/exception/exception-abstract.cpp.

include/dynamic-graph/exception-abstract.h

@@ -80,7 +80,7 @@ public:
   int getCode (void);
 
   /** Reference access to the error message (can be empty). */
-  const std::string &getStringMessage (void);
+  const std::string& getStringMessage (void) const;
 
   /** Access to the pointer on the array of  \e char related to the error string.
    * Cannot be  \e NULL.

src/exception/exception-abstract.cpp

@@ -54,8 +54,8 @@ getMessage (void)
     return (this->message) .c_str();
 }
 
-const string &ExceptionAbstract::
-getStringMessage (void)
+const string& ExceptionAbstract::
+getStringMessage (void) const
 {
     return this->message;
 }