Since Rmpi/pbdMPI pre-allocate memory to store comm, status,
datatype, info, request, this function provides a variable in R
to let different APIs share the same memory address.
If the package loads first, then this sets `.__MPI_APTS__
'
in the .GlobalEnv
of R. If the package does not load before other
MPI APIs, then this gives a structure pointer to external memory
according to `.__MPI_APTS__
', i.e. allocated by other MPI APIs.
pbdMPI/R/arrange.mpi.apts
provides the R code, and
pbdMPI/src/pkg_*.*
provides the details of this call.