A function to calculate outcomes for units in a particular time period `tp` in a panel data setting (this function can also be used to recover covariates, etc. in the first period).
get_Yit(df, tp, idname, yname, tname)a vector of outcomes in period t, the vector will have the length nT (i.e., this is returned for each element in the panel, not for a particular period)
the data.frame used in the function
The time period for which to get the outcome
name of column that holds the unit id
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