fix calculation for covariance with new stdev
fix calculation for covariance with new stdev

ggonzale/jstat,smarden1/jstat,jstat/jstat,ggonzale/jstat,akrawitz/jstat,cmpolis/jstat,jstat/jstat,akrawitz/jstat,JeebsM/jstat,jamescgibson/jstat,smarden1/jstat,JeebsM/jstat,NorthDecoder/jstat,NorthDecoder/jstat