Computes the quantile scores for the values in a group.
A quantile is a generic interval of user-defined width. For example, percentiles divide data among 100 evenly spaced intervals, deciles among 10 evenly spaced intervals, quartiles among 4, and so on. A quantile score indicates the fraction of rows having a sort_expression value lower than the current value. For example, a percentile score of 98 means that 98 percent of the rows in the list have a sort_expression value lower than the current value.