Added flatchain property to ptsampler.
Added flatchain property to ptsampler.

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