added chi-square, chi-square cdf and lower gamma functions
added chi-square, chi-square cdf and lower gamma functions

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