Write checkpoint when end simulation time has been reached (#1218)

Description
This small change ensures that a checkpoint is written if the end simulation time has been reached regardless of the frequency for the check pointing specified in the parameter file.

Testing
All test involving a restart should pass without any issue. I have tested the feature with an example on my local machine and it works as expected.

Documentation
No change needed in the documentation.

source/solvers/navier_stokes_base.cc

@@ -530,9 +530,10 @@ NavierStokesBase<dim, VectorType, DofsType>::finish_time_step()
     }
   if (this->simulation_parameters.restart_parameters.checkpoint &&
       simulation_control->get_step_number() != 0 &&
-      simulation_control->get_step_number() %
-          this->simulation_parameters.restart_parameters.frequency ==
-        0)
+      (simulation_control->get_step_number() %
+           this->simulation_parameters.restart_parameters.frequency ==
+         0 ||
+       simulation_control->is_at_end()))
     {
       this->write_checkpoint();
     }