devel/lwgrp: New port: Light-weight group library for MPI

The Light-weight Group library provides data structures and collectives to
create/destroy ordered process groups efficiently in MPI codes — useful when
frequent, transient groups make communicator creation too costly.

https://github.com/llnl/lwgrp/

It is one of the dependencies for sysutils/mpifileutils.

PR: 291587
Sponsored by: UNIS Labs
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>