ENH: Allow passing an existing engine to TradingEnvironment
ENH: Allow passing an existing engine to TradingEnvironment

Specifically to allow the use case of creating
an in-memory SQLite database and populating
it with assets before creating the trading
environment.

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