Major Description
The class Percentile only support estimation on non-weighted samples.
I've implement some function to estimate percentiles of weighted samples.
here is the reference: https://stats.stackexchange.com/questions/13169/defining-quantiles-over-a-weighted-sample
When all weights are equal to each other, it works like estimation on non-weighted samples under R-7.
I can't find formulas for other rules but at least, now it can evaluate percentile for weighted samples.
