hypergeometric random draws
[x]=hypg_rnd(nobs,n,K,N)
* nobs = number of draws
* n,K,N = parameters of the distribution
* x = a vector of random draws from the distribution
NOTE: mean = (n/N)*k ; variance = [(n*K)/(N*N)*(N-1)]*(N-K)*(N-n)
p=hypg_pdf(100,15,20,30)