xdmfHDF: Use UseSerialFile by default

This is done because, similarly to ioss library,
the vtkXDMFReader decides which rank reads which part of the dataset
and not the xdmf2 library itself.

ThirdParty/xdmf2/vtkxdmf2/libsrc/XdmfHDF.cxx

@@ -50,7 +50,7 @@ XdmfHDF::XdmfHDF() {
   // Defaults
   this->NumberOfChildren = 0;
   this->Compression = 0;
-  this->UseSerialFile = 0;
+  this->UseSerialFile = 1; // Default to Serial, because the vtkXdmfReader handles the parallelism.
   // We may have been compiled with Parallel IO support, but be run only on a single
   // machine without mpiexec. Disable parallel if just one process.
 #if defined(H5_HAVE_PARALLEL) && ((H5_VERS_MAJOR>1)||((H5_VERS_MAJOR==1)&&(H5_VERS_MINOR>=6)))