Dealing with Fenix_Init NULL output communicator in Fortran

[rfvander]

Inspired by the C interface of the Fenix library, we allow the user to specify NULL for the robust output communicator. This is not possible in Fortran. There is no error in the specification, since we have not yet defined the Fortran interface. I propose that we use MPI_COMM_NULL for Fortran, but keep NULL for C.