make_mpiP_work_with_MPI_IN_PLACE.md

Follow https://github.com/LLNL/mpiP for configuration. In the generated mpiP-wrappers.c, do following changes:

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1.In function static int mpiPif_MPI_Allgatherv(...)

mpiPi_stats_mt_enter(hndl);
++ if ( *sendtype == MPI_DATATYPE_NULL ) sendbuf = MPI_IN_PLACE;
rc = PMPI_Allgather( sendbuf, * sendcount, …)

2.In function static int mpiPif_MPI_Allgather

mpiPi_stats_mt_enter(hndl);
++ if ( *sendtype == MPI_DATATYPE_NULL ) sendbuf = MPI_IN_PLACE;
rc = PMPI_Allgather( sendbuf, * sendcount, ...);

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Then make and compile.