Output of Xdmf data in Fortran can now take advantage of xpointer functionality.
Calling any AddPrevious or SetPrevious methods will reuse previously
added Xdmf structure so that when it is written to disk xincludes can be
utilized rather than rewriting identical structure multiple times.  These
methods take an id which is returned in Add or Set calls.

Modified TestXdmfFortran.f90 to utilize new functionality.