Change checksum backend to h5py to improve CI performance (#725)

* Update enforcePeriodic call (change in AMReX)

* Test CI performance

* change openpmd-viewer backend in checksum to h5py

Author: AlexanderSinn

tests/checksum/backend/openpmd_backend.py

@@ -17,7 +17,7 @@ def __init__(self, filename):
         ''' Constructor: store the dataset object
         '''
 
-        self.dataset = OpenPMDTimeSeries(filename)
+        self.dataset = OpenPMDTimeSeries(filename, backend='h5py')
 
     def fields_list(self):
         ''' Return the list of fields defined on the grid