BUG: If we have compiled with Parallel IO and MPI, but are actually runnin a serial only process, the Parallel file access needs to be disabled. Add a check in the XdmfHDF constructor to set UseSerialFile if either MPI is not initialized, or nprocs<=1
BUG: If we have compiled with Parallel IO and MPI, but are actually runnin a serial only process, the Parallel file access needs to be disabled.
Add a check in the XdmfHDF constructor to set UseSerialFile if either MPI is not initialized, or nprocs<=1

cjh1/Xdmf2,cjh1/Xdmf2,cjh1/Xdmf2