fix calc problem in covariance
fix calc problem in covariance

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