MAINT: Revert slice into returns containers instead of using .valid()
MAINT: Revert slice into returns containers instead of using .valid()

Backing out slice vs. valid(), because of an incompatiblity with
starting a minutely emitted session mid-day, since the midday start
date is not yet wired through SimulationParameters.

bartosh/zipline,dmitriz/zipline,stkubr/zipline,ronalcc/zipline,DVegaCapital/zipline,jimgoo/zipline-fork,StratsOn/zipline,mattcaldwell/zipline,cmorgan/zipline,nborggren/zipline,keir-rex/zipline,sketchytechky/zipline,davidastephens/zipline,euri10/zipline,mattcaldwell/zipline,ChinaQuants/zipline,umuzungu/zipline,bartosh/zipline,AlirezaShahabi/zipline,morrisonwudi/zipline,enigmampc/catalyst,semio/zipline,jimgoo/zipline-fork,alphaBenj/zipline,humdings/zipline,ronalcc/zipline,wubr2000/zipline,otmaneJai/Zipline,jordancheah/zipline,magne-max/zipline-ja,joequant/zipline,CarterBain/AlephNull,michaeljohnbennett/zipline,chrjxj/zipline,quantopian/zipline,semio/zipline,aajtodd/zipline,dkushner/zipline,ChinaQuants/zipline,euri10/zipline,magne-max/zipline-ja,grundgruen/zipline,CDSFinance/zipline,Scapogo/zipline,Scapogo/zipline,dhruvparamhans/zipline,davidastephens/zipline,AlirezaShahabi/zipline,erikness/AlephOne,kmather73/zipline,grundgruen/zipline,wubr2000/zipline,wilsonkichoi/zipline,quantopian/zipline,chrjxj/zipline,zhoulingjun/zipline,CarterBain/AlephNull,florentchandelier/zipline,YuepengGuo/zipline,keir-rex/zipline,jordancheah/zipline,StratsOn/zipline,otmaneJai/Zipline,cmorgan/zipline,dhruvparamhans/zipline,joequant/zipline,kmather73/zipline,iamkingmaker/zipline,CDSFinance/zipline,MonoCloud/zipline,michaeljohnbennett/zipline,alphaBenj/zipline,gwulfs/zipline,enigmampc/catalyst,gwulfs/zipline,wilsonkichoi/zipline,aajtodd/zipline,dmitriz/zipline,MonoCloud/zipline,DVegaCapital/zipline,zhoulingjun/zipline,sketchytechky/zipline,dkushner/zipline,stkubr/zipline,florentchandelier/zipline,humdings/zipline,umuzungu/zipline,morrisonwudi/zipline,YuepengGuo/zipline,iamkingmaker/zipline,nborggren/zipline,erikness/AlephOne