Helper function to compute the upper bound in bounds method. It is usually called by vapply function
getub(s, F.change.treated, F.treated.tmin1, y)
A particular value of distribution for which to calculate the bound
ecdf object of distribution of change in outcomes for the treated group
ecdf object of distribution of outcomes in period t-1 for the treated group
a vector of values that observations could take in the previous period ?