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/neigh_list.cpp

@@ -79,6 +79,7 @@ NeighList::NeighList(LAMMPS *lmp) : Pointers(lmp)
   // USER-DPD package
 
   ndxAIR_ssa = NULL;
+  np = NULL;
 }
 
 /* ---------------------------------------------------------------------- */

src/neigh_list.h

@@ -82,6 +82,7 @@ class NeighList : protected Pointers {
 
   int AIRct_ssa[8]; // count of how many atoms in each AIR
   uint16_t (*ndxAIR_ssa)[8]; // for each atom, last neighbor index of each AIR
+  class NPair *np;           // ptr to NPair instance I depend on
 
   // methods
 

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;