增加权重筛选的功能

var data=new List<WeightedItem<string>>()
{
     new WeightedItem<string>("A", 1),
     new WeightedItem<string>("B", 3),
     new WeightedItem<string>("C", 4),
     new WeightedItem<string>("D", 4),
};
var item=data.WeightedItem();//按权重选出1个元素
var list=data.WeightedItems(2);//按权重选出2个元素

var selector = new WeightedSelector<string>(new List<WeightedItem<string>>()
{
    new WeightedItem<string>("A", 1),
    new WeightedItem<string>("B", 3),
    new WeightedItem<string>("C", 4),
    new WeightedItem<string>("D", 4),
});
var item = selector.Select();//按权重选出1个元素
var list = selector.SelectMultiple(3);//按权重选出3个元素