NDArray: Rename variable
NDArray: Rename variable

Rename variable to make it clear that it refers to values for a single
model variable rather than a trace object.

https://github.com/pymc-devs/pymc3/pull/771#discussion_r32848583

jameshensman/pymc3,Anjum48/pymc3,hothHowler/pymc3,wanderer2/pymc3,MichielCottaar/pymc3,tyarkoni/pymc3,MichielCottaar/pymc3,superbobry/pymc3,tyarkoni/pymc3,hothHowler/pymc3,CVML/pymc3,superbobry/pymc3,arunlodhi/pymc3,clk8908/pymc3,arunlodhi/pymc3,wanderer2/pymc3,dhiapet/PyMC3,dhiapet/PyMC3,Anjum48/pymc3,jameshensman/pymc3,kmather73/pymc3,clk8908/pymc3,CVML/pymc3,JesseLivezey/pymc3,kmather73/pymc3,evidation-health/pymc3,JesseLivezey/pymc3,evidation-health/pymc3