subdag operator

This README demonstrates the use of the subdag operator.

The subdag operator allows you to effectively run a driver within a node. In this case, we are calculating unique website visitors from the following set of parameters:

1. Region = CA (canada) or US (United States)
2. Granularity of data = (day, week, month)

You can find the code in unique_users.py and reusable_subdags.py and look at how we run it in main.py.