Make grackle verbose only for rank 0 in MPI runs

src/cooling/grackle/cooling.c

@@ -1280,8 +1280,10 @@ void cooling_init_units(const struct unit_system* us,
 void cooling_init_grackle(struct cooling_function_data* cooling) {
 
 #ifdef SWIFT_DEBUG_CHECKS
-  /* enable verbose for grackle */
-  grackle_verbose = 1;
+  /* Enable verbose for grackle for rank 0 only. */
+  if (engine_rank == 0) {
+    grackle_verbose = 1;
+  }
 #endif
 
   chemistry_data* chemistry = &cooling->chemistry_data;