Fixing group names in MGXS HDF5 file (#3708)

Author: shimwell

openmc/mgxs/mgxs.py

@@ -1923,7 +1923,7 @@ def build_hdf5_store(self, filename='mgxs.h5', directory='mgxs',
 
             # Create an HDF5 group for the subdomain
             if self.domain_type == 'distribcell':
-                group_name = ''.zfill(num_digits)
+                group_name = str(subdomain).zfill(num_digits)
                 subdomain_group = domain_group.require_group(group_name)
             else:
                 subdomain_group = domain_group