This is a temporary function, could be merged into the main function condKendall.
Give conditional Kendall's tau with the ability to take on perturbation weights.
wKendall(trun, obs, delta = NULL, weights = NULL)left truncation time satisfying trun <= obs.
observed failure time, must be the same length as trun, might be right-censored.
an optional 0-1 vector of censoring indicator (0 = censored, 1 = event) for obs.
If this vector is not specified, condKendall assumes no censoring and all observed failure time
denote events.
perturbation weights.