Check that hdf5 has MPI if performing depletion with MPI

Check added in openmc/depletion/__init__.py.
Without this check, the exporting of the Results to hdf5 will
hang, as the second process attempts to write to a file
that has already been opened on another process.
This error is only raised after a full transport calculation
has been run.
The check raises a more helpful error directly at the import
from openmc.deplete, prior to transport calculations.

openmc/deplete/__init__.py

@@ -6,10 +6,24 @@ A depletion front-end tool.
 """
 
 from .dummy_comm import DummyCommunicator
+
 try:
     from mpi4py import MPI
+
     comm = MPI.COMM_WORLD
     have_mpi = True
+    # check if running with MPI and if hdf5 is MPI-enabled
+    from h5py import get_config
+
+    if not get_config().mpi and comm.size > 1:
+        # Raise exception only on process 0
+        if comm.rank:
+            from sys import exit
+
+            exit()
+        raise RuntimeError(
+            "Need MPI-enabled HDF5 install to perform depletion with MPI"
+        )
 except ImportError:
     comm = DummyCommunicator()
     have_mpi = False