replace relay mpi C99 allocs with c++ style allocs

[cyrush]

    int intArray[2];
    MPI_Status status;

    int mpi_error = MPI_Recv(intArray, 2, MPI_INT, src, tag, comm, &status);
    CONDUIT_CHECK_MPI_ERROR(mpi_error);

    int schema_len = intArray[0];
    int data_len = intArray[1];

    char schema[schema_len + 1];
    char data[data_len + 1];

the above allocs for schema and data are working -- but they aren't typical C++ allocs. We should replace them