TL: fixed the fieldsIO when mpi4py not available

Author: tlunet

pySDC/helpers/fieldsIO.py

@@ -10,7 +10,8 @@
 try:
     from mpi4py import MPI
 except ImportError:
-    pass
+    class MPI:
+        COMM_WORLD = None
 
 T = TypeVar("T")