reset the timer also on non-root MPI processes (#1787)

Co-authored-by: Michael Schlottke-Lakemper <michael@sloede.com>

src/callbacks_step/summary.jl

@@ -152,7 +152,13 @@ function initialize_summary_callback(cb::DiscreteCallback, u, t, integrator;
         Polyester.reset_threads!()
     end
 
-    mpi_isroot() || return nothing
+    # The summary callback should only print information on the root process.
+    # However, all other MPI processes should also reset the timer so that
+    # it can be used to diagnose performance.
+    if !mpi_isroot()
+        reset_timer!(timer())
+        return nothing
+    end
 
     print_startup_message()