PERF: Performance improvements to regression/correlation.
PERF: Performance improvements to regression/correlation.

- ~7x speedup on PearsonR.
- ~30x speedup on SpearmanR (this will still be tight for hitting a 5
  minute compute timeout on Quantopian).
- ~25% speedup on LinearRegression.

humdings/zipline,quantopian/zipline,bartosh/zipline,bartosh/zipline,wilsonkichoi/zipline,magne-max/zipline-ja,alphaBenj/zipline,enigmampc/catalyst,magne-max/zipline-ja,Scapogo/zipline,wilsonkichoi/zipline,humdings/zipline,florentchandelier/zipline,enigmampc/catalyst,alphaBenj/zipline,florentchandelier/zipline,Scapogo/zipline,quantopian/zipline