Implement function to measure effect of appearing/disappearing products

[grahamjwhite]

Using the method of Feenstra (1994), and the function evaluateMatched(), implement another function to estimate the effect of appearing and disappearing products on the CES price index.

Feenstra (1994), "New Product Varieties and the Measurement of International Prices", The American Economic Review, Vol. 84, No. 1 (Mar., 1994), pp. 157-177.