Takes a data.frame and computes for a particular group g and time period t and computes an estimate of a group time average treatment effect and a corresponding influence function using a difference in differences approach.
The code relies on gt_data having certain variables defined.
In particular, there should be an id column (individual identifier),
D (treated group identifier), period (time period), name
(equal to "pre" for pre-treatment periods and equal to "post" for post
treatment periods), Y (outcome).
In our case, we call two_by_two_subset which sets up the
data to have this format before the call to did_attgt.