MAINT: Group by loader and extra_rows
MAINT: Group by loader and extra_rows

so that the mask and dates are the same for all the columns
the loader is loading at a time.

umuzungu/zipline,humdings/zipline,magne-max/zipline-ja,quantopian/zipline,florentchandelier/zipline,alphaBenj/zipline,ChinaQuants/zipline,florentchandelier/zipline,enigmampc/catalyst,Scapogo/zipline,nborggren/zipline,quantopian/zipline,alphaBenj/zipline,dmitriz/zipline,wilsonkichoi/zipline,magne-max/zipline-ja,dmitriz/zipline,bartosh/zipline,grundgruen/zipline,humdings/zipline,grundgruen/zipline,enigmampc/catalyst,wilsonkichoi/zipline,jimgoo/zipline-fork,jimgoo/zipline-fork,ChinaQuants/zipline,nborggren/zipline,bartosh/zipline,umuzungu/zipline,Scapogo/zipline