Create unique id for an MPI file handle about fsprj2

[PhilKoe]

A possible solution could be to use the Fortran file handle instead of C structure. The Fortran handle is a single integer. With the function MPI_Fint MPI_File_c2f(MPI_File file) it is possible to convert a MPI_File structure to such an integer (see "MPI: A Message-Passing Interface Standard Version 3.1" page 654 - 655). This function could be implemented as a macro (see page 20).

In OpenMPI MPI_File_c2f is a function that extracts the integer from an element field in the MPI_File structure (see https://github.com/open-mpi/ompi/blob/master/ompi/mpi/c/file_c2f.c).