[Lumley] Write an R function that takes inputs n1, n2, N1, N2,
σ12,σ2and computes the variance of the population total in a
stratified sample. Choose some reasonable values of the population
sizes and variances, and graph this function as n1 and n2 change,
to find the optimum and to examine how sensitive the variance is
the precise values of n1 and n2.