define MPI constants in mpi_wrapper.c (or mpi_constants.c) once renamed as constants

[gxyd]

Description

Most (or maybe all):

fortran_mpi/src/mpi_wrapper.c

Lines 3 to 17 in 90aca68

	MPI_Status* c_MPI_STATUSES_IGNORE = MPI_STATUSES_IGNORE;
	MPI_Info c_MPI_INFO_NULL = MPI_INFO_NULL;
	MPI_Comm c_MPI_COMM_WORLD = MPI_COMM_WORLD;
	MPI_Datatype c_MPI_DOUBLE = MPI_DOUBLE;
	MPI_Datatype c_MPI_FLOAT = MPI_FLOAT;
	MPI_Datatype c_MPI_INT = MPI_INT;
	void* c_MPI_IN_PLACE = MPI_IN_PLACE;
	MPI_Op c_MPI_SUM = MPI_SUM;

of the variables can be prefixed with const

[gxyd]

Once the PR: #117 is merged (i.e. renaming mpi_wrapper.c to mpi_constants.c), we can probably work on this issue, otherwise it will cause merge conflicts