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Change using_mpi test

Sylvia Biscoveanu requested to merge sylvia.biscoveanu/bilby:multinest_mpi into master

I've run into a new bug with the current bilby implementation of pymultinest. There is a new environment variable that comes up by default called VT_MPI, which triggers this clause to always set using_mpi to True. I've changed this clause to instead check that MPI.COMM_WORLD.Get_size()>1. I've tested this with a simple Hello World script as follows:

from mpi4py import MPI
import sys
import os

size = MPI.COMM_WORLD.Get_size()
rank = MPI.COMM_WORLD.Get_rank()
name = MPI.Get_processor_name()
using_mpi = (MPI.COMM_WORLD.Get_size()>1)
print(using_mpi)

sys.stdout.write(
    "Hello, World! I am process %d of %d on %s.\n"
    % (rank, size, name))

which produces the following output

> python test_mpi.py 
False
Hello, World! I am process 0 of 1 on ldas-pcdev3.
> mpiexec -n 4 python test_mpi.py 
True
Hello, World! I am process 1 of 4 on ldas-pcdev3.
True
Hello, World! I am process 3 of 4 on ldas-pcdev3.
True
Hello, World! I am process 2 of 4 on ldas-pcdev3.
True
Hello, World! I am process 0 of 4 on ldas-pcdev3.

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