Fix problem when checkpoint_handler is none

hysop/problem.py

-import sys, datetime
+import sys
+import datetime
 
 from hysop.constants import Backend, MemoryOrdering
 from hysop.tools.types import check_instance, first_not_None, to_tuple, to_list
@@ -15,7 +16,7 @@ class Problem(ComputationalGraph):
 
     def __init__(self, name=None, method=None, mpi_params=None,
                  check_unique_clenv=True, **kwds):
-        mpi_params = first_not_None(mpi_params, MPIParams()) # enforce mpi params for problems
+        mpi_params = first_not_None(mpi_params, MPIParams())  # enforce mpi params for problems
         super(Problem, self).__init__(name=name, method=method, mpi_params=mpi_params, **kwds)
         self._do_check_unique_clenv = check_unique_clenv
 
@@ -138,8 +139,9 @@ class Problem(ComputationalGraph):
         simu.initialize()
 
         check_instance(checkpoint_handler, CheckpointHandler, allow_none=True)
-        checkpoint_handler.create_checkpoint_template(self, simu)
-        checkpoint_handler.load_checkpoint(self, simu)
+        if checkpoint_handler:
+            checkpoint_handler.create_checkpoint_template(self, simu)
+            checkpoint_handler.load_checkpoint(self, simu)
 
         vprint('\nSolving problem...')
         with Timer() as tm:
@@ -147,7 +149,7 @@ class Problem(ComputationalGraph):
                 vprint()
                 simu.print_state()
                 self.apply(simulation=simu, dbg=dbg, **kwds)
-                should_dump_checkpoint = checkpoint_handler.should_dump(simu) # determined before simu advance
+                should_dump_checkpoint = checkpoint_handler and checkpoint_handler.should_dump(simu)  # determined before simu advance
                 simu.advance(dbg=dbg, plot_freq=plot_freq)
                 if should_dump_checkpoint:
                     checkpoint_handler.save_checkpoint(self, simu)
@@ -166,7 +168,8 @@ class Problem(ComputationalGraph):
         vprint_banner(msg, spacing=True, at_border=2)
 
         simu.finalize()
-        checkpoint_handler.finalize(self.mpi_params)
+        if checkpoint_handler:
+            checkpoint_handler.finalize(self.mpi_params)
         self.final_report()
 
         if (dbg is not None):
@@ -179,4 +182,3 @@ class Problem(ComputationalGraph):
     def finalize(self):
         vprint('Finalizing problem...')
         super(Problem, self).finalize()
-