Calculates a units outcome in some particular period `tp`. This function operates on a data.frame that is already local to a particular unit.
get_Yit_inner(this_df, tp, yname, tname)
a data.frame, for this function it should be specific to a particular unit
The time period for which to get the outcome
name of column containing the outcome (or other variable) for which to calculate its outcome in the immediate pre-treatment period
name of column that holds the time period