很多时候我们知道如何对一个矢量建立一个缓冲区的，但是不知道如何对矢量集合进行批量缓冲区分析，所以这里我们需要建立一个循环，也就是用map进行分析，对每一个矢量分别进行操作。这里我们还需要注意一点。最后我们需要将每一个矢量转化为其几何体，这样就可以进行图层加载和展示了。
函数：
map(algorithm, dropNulls)
Maps an algorithm over a collection.
Returns the mapped collection.
Arguments:
this:collection (Collection):
The Collection instance.
algorithm (Function):
The operation to map over the images or features of the collection. A JavaScript function that receives an image or features and returns one. The function is called only once and the result is captured as a description, so it cannot perform imperative operations or rely on external state.
dropNulls (Boolean, optional):
If true, the