Feature/make add pair protected

The add_collision_pair is changed from private to protected as also potential children should be able to access it.

cpp/particles/p2p/p2p_ghost_collisions.hpp

@@ -55,6 +55,7 @@ class p2p_ghost_collisions
         // description for disks:
         double disk_width;
         bool isActive;
+   protected:        
         void add_colliding_pair(partsize_t idx_part1, partsize_t idx_part2)
         {
             // store colliding particle ids in order, to be able to identify pairs more easily
@@ -69,7 +70,6 @@ class p2p_ghost_collisions
             this->collision_pairs_local.push_back(idx_part_small);
             this->collision_pairs_local.push_back(idx_part_big);
         }
-    protected:
         bool synchronisation;
         std::vector <partsize_t> collision_pairs_local;
         std::vector <partsize_t> collision_pairs_global;