函数：

aggregate_sum(property)

Aggregates over a given property of the objects in a collection, calculating the sum of the values of the selected property.

对一个集合中的对象的特定属性进行聚合，计算所选属性值的总和。

Arguments:

this:collection (FeatureCollection):

The collection to aggregate over.

property (String):

The property to use from each element of the collection.

Returns: Number