Add 2 references to MPI_COMM_SPLIT_TYPE

[GuillaumeMercier]

Problem

Add two references to MPI_COMM_SPLIT_TYPE, dealing with HW communicators.

Proposal

Changes to the Text

Add a couple of lines in the last advice to implementors (non-normative text) with the references.

Impact on Implementations

None.

Impact on Users

None.

References

Here are the references in question:

@inproceedings{Traff03:archaware,
Author={Jesper Larsson Tr{"a}ff},
Title="{SMP}-Aware Message Passing Programming",
Booktitle={Eigth International Workshop on High-level Parallel Programming Models and Supportive Environments
{(HIPS)}, 17th International Parallel and Distributed Processing Symposium {(IPDPS)}},
Year={2003},
Pages={56--65}
}

and

@Article{Mercier18:hsplit,
author = {Brice Goglin and Emmanuel Jeannot and Farouk Mansouri and Guillaume Mercier},
title = {Hardware topology management in {MPI} applications through hierarchical communicators},
journal = {Parallel Computing},
volume = {76},
pages = {70--90},
year = {2018},
doi = {10.1016/j.parco.2018.05.006},
}