MAINT: Factor out risk metric logic from risk objects.
MAINT: Factor out risk metric logic from risk objects.

Move the risk metric definitions to functions at the module level
with defined parameters.

Both risk implementations call these functions, where the difference
between risk implementations is with which internal data they
send to the various risk metrics.

Metrics moved:
- Sharpe Ratio
- Sortino Ratio
- Information Ration
- Alpha

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