ENH: Add methods in Fortran interface to reuse previously added data.
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.