Added unittest for dead walkers (initialized with lnprob0 = NaN).
Added unittest for dead walkers (initialized with lnprob0 = NaN).

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