Disable MPI writer on single core

src/HDF5_wr/HDF5_writer_vd.hpp

src/VTKWriter/VTKWriter.hpp

src/VTKWriter/VTKWriter_grids_util.hpp

src/VTKWriter/VTKWriter_point_set.hpp

src/VTKWriter/VTKWriter_unit_tests.hpp

src/VTKWriter/byteswap_portable.hpp

src/VTKWriter/is_vtk_writable.hpp