Two-step price index calculation

[grahamjwhite]

Include a method for stepwise price index calculation (ID1, ID2)
First step: elementary aggregates (Jevons, Carli, Dutot) per ID1 (e.g. product class) times the overall expenditure share of the elements of the class
Second step: aggregate by a second ID2 (some division, where several ID1 belong to one ID2) to generate a matrix containing the price indices and the aggregated weights of the containing ID1's for all t

[grahamjwhite]

#23

Separating this issue out from the Geometric Laspeyres and Paasche index enchancement.