Non-extensive measures e.g. COUNT DISTINCT

[danfrankj]

Given an input "transaction fact" containing one row per transaction, compute "transacting users by day".

In SQL this might accomplished via

SELECT 
  COUNT(DISTINCT user_id)
  , ts::DAY
FROM transaction_fct
GROUP BY 
  ts::DAY  

In particular to compute this we need not join any table containing all users as this may be quite expensive.

Potentially related to #30