[Feature Request] Get all point cloud/sfm clusters

Is your feature request related to a problem? Please describe.
Some users may want to retrieve the point clouds for a geographic. Each Mapillary image has an associated point cloud in the sfm_cluster field.

Describe the solution you'd like
There are two ways to allow a user to retrieve this:

1. a list of links
2. a list of point cloud objects

The function could be something like get_point_clouds(bbox,links=false) which does the following:

1. calls graph.mapillary.com/images?bbox=lon1,lat1,lon2,lat2&limit=5000
2. checks if bbox includes 5000 or more response items (if so, it is maxxed out, we need smaller bbox)
3. if not maxxed out, get all the images in the bbox with fields=sfm_cluster
4. if links=false (default) run a loop, for each image, do a get request for the sfm_cluster URL, and append to a list
5. return the list of sfm_cluster data objects
6. if links=true, then just return a list of the URLs, do not request the data
7. if bbox too big in step 2, then split into 4 smaller bboxes, or warn user to simply use a smaller bbox

Describe alternatives you've considered
It might be good to just request using the tiles, the way we get images from a bbox, but I am not certain that these images via tiles have the sfm_cluster URL, so it would not work

Additional context
The point cloud should be in .ply file format, but this is not important, just retrieving the objects lets the user make use of them (they would use it with the OpenSfM library)

从 SDK 现有的 bounding-box 图像检索功能和 graph.mapillary.com/images endpoint 开始。验证 sfm_cluster 的暴露方式，然后定义 get_point_clouds(bbox, links=false) 在返回 links 还是已获取对象，以及处理过大 box 时的行为。完成标准是：对于有效的 bbox 返回请求的 point-cloud 结果，并明确处理 5000 项的限制。