Update combination.h

src/ibea/combination.h

 #ifndef COMBINATION_H
 #define COMBINATION_H
 
-double simple_combination(double parent,
+void simple_combination(Population & pop_in,
+                          Population & pop_out,
+                          int parent1,
+                          int parent2,
                           const double eta,
                           coco_random_state_t *random_generator);