Only delete compcomm on icomm if it exists

Within the MPI implementation, an inner communicator can optionally
contain a compilation commucator. An indentation error meant that the
compilation attribute was unconditionally deleted, causing incorrect
MPI behaviour if the attribute had never been set in the first
place. On MPICH, this doesn't seem to raise any errors, but with
OpenMPI it causes errors like:

    mpi4py.MPI.Exception: MPI_ERR_OTHER: known error not in list

pyop2/mpi.py

@@ -201,7 +201,7 @@ def delcomm_outer(comm, keyval, icomm):
     if comp_comm is not None:
         debug('Removing compilation comm on inner comm')
         decref(comp_comm)
-    icomm.Delete_attr(compilationcomm_keyval)
+        icomm.Delete_attr(compilationcomm_keyval)
 
     # Once we have removed the reference to the inner/compilation comm we can free it
     cidx = icomm.Get_attr(cidx_keyval)