[#2543] quantityIndex() with EKS transformation

In my own research I found a need to compute transitive Fisher quantity indices for millions of farms. Naive application of the formula for the EKS correction for N firms requires the computation of N^2 quantity indices. To make computation feasible, I wrote a function with RcppArmadillo that takes advantages of cases where firms have zeros in their quantity vector or face the same price vector as other firms.

Would you be interested in me contributing this function as a subfunction of quantityIndex()? Enabling computation of the Laspeyres and Paasche indices would be trivial. The only "cost" of this function would be that it would require micEcon to list RcppArmadillo as an imported package.