deal with MPI comm_world when mpi4py not installed
deal with MPI comm_world when mpi4py not installed

pkgw/emcee,dfm/emcee3,joezuntz/emcee,dfm/emcee,linan7788626/emcee,ipashchenko/emcee,farr/emcee,jellis18/emcee3,ga7g08/emcee,drphilmarshall/emcee,elkingtonmcb/emcee,willvousden/emcee,johnbachman/emcee,johnbachman/emcee,willvousden/emcee,dfm/emcee