A function that takes an original data set and keeps all pre-treatment data for all groups. For group g, it also includes data for the current period.
Also, note that if tp is still a pre-treatment period for group g,
then periods after tp will also be dropped for group g. This is a
design choice and is useful especially for estimating placebo
group-time average treatment effects in pre-treatment periods.
A main use case for this function is to compute ATT(g,t)'s using a global estimation strategy such as imputation in Gardner (2022).