public static <T, U> Collector<T, ?, Optional<U>> median(Function<? super T, ? extends U> function, Comparator<? super U> comparator) {
        return percentile(0.5, function, comparator);
    }
Get a {@link Collector} that calculates the <code>MEDIAN()</code> function given a specific ordering.
jOOL-java-8/src/main/java/org/jooq/lambda/Agg.java