Fix test void to work with threading and mpi

This PR addresses the issue of answers in test_void not being reproducible when both threading and mpi are being used. A better starting keff was achieved by starting with a point source instead of box source. This is explained further in #271. 

The reviewer of this pull request should run omp tests with at least 4 threads, because this is what will probably be the number that will be used in the future for daily testing.