protect Dna_7 dtor

src/libaevol/7/Dna_7.h

@@ -52,9 +52,9 @@ class Dna_7 {
   friend class DnaFactory;
  protected:
   Dna_7(int length);
- public:
   ~Dna_7();
 
+ public:
     void set_indiv(Dna_7* dna, Individual_7* indiv);
     void set_indiv(Individual_7* indiv);
     void set_indiv(Dna* dna);