Issue77 Add evaluation contexts, to be passed into the evaluators by the search methods

src/search/pref_evaluator.h

 #ifndef PREF_EVALUATOR_H
 #define PREF_EVALUATOR_H
 
 #include "scalar_evaluator.h"
 
 #include <string>
 #include <vector>
 
 
 class PrefEvaluator : public ScalarEvaluator {
 private:
     bool value_preferred;
-
+protected:
+    void evaluate(int g, bool preferred);
+    bool is_last_evaluated_dead_end() const;
+    int get_last_evaluated_value() const;
 public:
     PrefEvaluator();
     ~PrefEvaluator();
 
-    void evaluate(int g, bool preferred);
-    bool is_dead_end() const;
     bool dead_end_is_reliable() const;
-    int get_value() const;
     void get_involved_heuristics(std::set<Heuristic *> &) {return; }
 };
 
 #endif