Compute the design according to unequal weighting.
a numeric vector with weights
The function will return the the kish factor
The factor is computed acording to 'Weighting for Unequal P_i', Leslie Kish, Journal of Official Statistics, Vol. 8. No. 2, 1992 $$ deff = \sqrt n \sum_j w_j^2 / (\sum_j w_j)^2$$
#>  1#>  1.273886