refactoring of USER-TALLY computes to handle sparse and hybrid systems

with sparse and hybrid systems, Pair::ev_tally() may not be called on
every processor and thus the computes in USER-TALLY may hang during
reverse communication because of the error->all() call after checking
whether callback from Pair::ev_tally() has been called at least once.
To address this cleanly, a second callback function needs to be added,
which is run during Pair::ev_setup() and will now handle all memory
re-allocation and clearing of accumulators, just like it is done for
regular tallied data.

src/pair.cpp

@@ -814,6 +814,16 @@ void Pair::ev_setup(int eflag, int vflag, int alloc)
     if (vflag_atom == 0) vflag_either = 0;
     if (vflag_either == 0 && eflag_either == 0) evflag = 0;
   } else vflag_fdotr = 0;
+
+
+  // run ev_setup option for USER-TALLY computes
+
+  if (num_tally_compute > 0) {
+    for (int k=0; k < num_tally_compute; ++k) {
+      Compute *c = list_tally_compute[k];
+      c->pair_setup_callback(eflag,vflag);
+    }
+  }
 }
 
 /* ----------------------------------------------------------------------