Prohibit the client from calling some constructors without arguments.

The reason is to avoid the misuse of constructors that should be called
with arguments.

mert/ScoreData.h

@@ -28,6 +28,9 @@ protected:
   name2idx arrayname2idx_; // map from name to index of array
 
 private:
+  // Do not allow the user to instanciate without arguments.
+  ScoreData() {}
+
   Scorer* theScorer;
   std::string score_type;
   size_t number_of_scores;

mert/mert.cpp

@@ -113,6 +113,9 @@ class OptimizationTask : public Moses::Task
   }
 
  private:
+  // Do not allow the user to instanciate without arguments.
+  OptimizationTask() {}
+
   Optimizer* m_optimizer;
   Point m_point;
   statscore_t m_score;