Fixes serial vs parallel HDF5 issue

We now set HDF5_PREFER_PARALLEL to true in order to make sure that the
correct parallel version of HDF5 is used on systems where also a serial
version is installed.

This does the trick on Ubuntu 18.04 and Debian GNU/Linux 9.12.

CMakeLists.txt

@@ -43,6 +43,7 @@ endif()
 
 if ( HYTEG_BUILD_WITH_XDMFWRITER )
     message(STATUS "Using XDMF_WRITER_ROOT = " $ENV{XDMF_WRITER_ROOT} )
+    set( HDF5_PREFER_PARALLEL "YES" )
     find_package( HDF5 REQUIRED )
     include_directories ( ${HDF5_INCLUDE_DIRS} )
     list ( APPEND SERVICE_LIBS ${HDF5_LIBRARIES} )