Derived datatypes

CombBLAS is full of struct MPI commitments of the form:

    struct mystruct
    {
        IT ind;
        NT num;
    };

    MPI_Datatype datatype;
    MPI_Type_contiguous(sizeof(mystruct), MPI_CHAR, &datatype );
    MPI_Type_commit(&datatype);

Based on my reading, these all should fail: https://stackoverflow.com/questions/9864510/struct-serialization-in-c-and-transfer-over-mpi

How come CombBLAS work 99% of the time? I think it is mostly due to this reality: https://software.intel.com/en-us/forums/intel-visual-fortran-compiler-for-windows/topic/285999#comment-1504097

Time to replace our MPI_Type_contiguous with MPI_Type_structs? (or some simpler options like MPI_Type_indexed_block)

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