R.QBETA(p,a,b,lower_tail,log_p)
p: probability
a: the first shape parameter of the distribution
b: the second scale parameter of the distribution
lower_tail: if true (the default), the lower tail of the distribution is considered
log_p: if true, log of the probability is used
This function returns the probability quantile function, i.e., the inverse of the cumulative distribution function, of the beta distribution.