Corrected a bug in iact_self_direct() that erased all previously computed accelerations.

examples/test_bh_sorted.c

@@ -1491,7 +1491,7 @@ void iact_self_direct(struct cell *c) {
 
     /* Copy the local particle data back. */
     for (k = 0; k < count; k++) {
-      for (j = 0; j < 3; j++) c->parts[k].a[j] = parts[k].a[j];
+      for (j = 0; j < 3; j++) c->parts[k].a[j] += parts[k].a[j];
     }
   } /* otherwise, compute interactions directly. */
 }