comet files in simfactory use one MPI rank per node

The current (https://bitbucket.org/simfactory/simfactory2/src/master/mdb/machines/comet.ini) uses 1 MPI rank per node:

max-num-threads = 24
num-threads     = 24

This is usually not the best way to set things up, I would eg have expected that the default choice would be something like 1 MPI rank per NUMA domain. Given that, unless limited by communication overhead, we seem to obtain fastest per-node performance when using only MPI and no OpenMP (about a factor of 50% speedup on my 12 core workstation with 2 NUMA domains) if anyone is using Comet for production work and wants to contribute their machine description file that would be great.

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