Updated ee.Image.cumulativeCost to be able to handle multi-band cost images.
使用数据：
Copernicus DEM GLO-30: Global 30m Digital Elevation Model
有五个波段：
cumdem
Image (5 bands)
type: Image
bands: List (5 elements)
0: “DEM”, double, EPSG:4326, 3x3 px
1: “EDM”, double, EPSG:4326, 3x3 px
2: “FLM”, double, EPSG:4326, 3x3 px
3: “HEM”, double, EPSG:4326, 3x3 px
4: “WBM”, double, EPSG:4326, 3x3 px
重点介绍一下这个函数，主要是是为了去除多波段影像中的其中多波段影像值全部变为0，
也就是你输入这么多波段，可以选择其中一个波段，作为映射区域，然后让其去掉该区域影像值，让其变为0。
cumulativeCost(source, maxDistance, geodeticDistance)
Computes a cumulative cost map based on an image containing costs to traverse each pixel and an image containing source locations.
Each output band represents the cumulative cost over the corresponding input cost band.
Arguments:
this:cost (Image):
An image representing the cost