Change LF stat "empirical accuracy" to "precision"

[astarostap]

I was using Metal's LF analysis function, which gives you the "empirical accuracy" of your LFs. It was pretty confusing, I wasn't sure if it was computing its accuracy or its precision. After some experimentation I realized it's computing rule precisions. So, I thought maybe it would be better to call it "precision" to avoid confusions.

[bhancock8]

Duplicate question answered in Snorkel repo: snorkel-team/snorkel#1435