Function which calculates for k=1, ..., K, Pr(log-HR >= lhr_null | theta.hat.k = lhr_con.k)
i.e. the posterior probability the true OS log-hr exceeds the minimum unacceptable
OS log-HR given the estimate of the log-hr at analysis k equals lhr_con.k (i.e. the estimate
is equal to the stage k 'continuation threshold').