USER-DPD: Save pointer to the NPair used to create the NeighList

Gives a user of NeighList access to data stored in a custom NPair

src/neighbor.cpp

@@ -785,7 +785,7 @@ int Neighbor::init_pair()
     }
 
     PairCreator pair_creator = pairclass[flag-1];
-    neigh_pair[i] = pair_creator(lmp);
+    lists[i]->np = neigh_pair[i] = pair_creator(lmp);
     neigh_pair[i]->post_constructor(requests[i]);
     neigh_pair[i]->istyle = flag;