Function parallel_max 

pub fn parallel_max<T, I, F, V>(items: I, extract: F) -> Option<V>
where
    T: Send,
    V: Send + Ord + Copy,
    I: ParallelIterator<Item = T>,
    F: Fn(&T) -> V + Sync + Send,

Find maximum value in parallel.

Returns None if the iterator is empty.