add a function to calculate index sizes for a query/queryset

Total branch size for a query/query set gives a good idea of a typical node cache size that would avoid branch misses (with scaling depending on whether it is decompressed).

It would be useful to have a function that can be run on a query/queryset which returned information about the number of leaves/nodes in each index that was used, and the total for the query/queryset.

This would provide useful information on estimating the effectiveness of the node cache, and the suggestions for sizing and effectiveness of the page and hpc-cache on the system.