getChildren method for getting children of a cluster on the next zoom

[mourner]

Fixes #13. This is a fast approach replacing changes proposed in #27 and #30. Also addresses mapbox/mapbox-gl-js#3318.

It pretty much doesn't affect performance while allowing to accurately fetch "children" clusters, either for spiderfying, generating a hull, precise "zoom to cluster" or any other features.

The way it works is storing the index of the "origin" point the cluster was created from as its id, and also storing parentId for each point/cluster that was clustered. Then you can fetch the "origin" point, do a radius query around it on the next zoom and filter out points that are not related to the cluster. This operation is almost instant since it's O(log N).

Let me know what you think!

cc @apkoponen @bewithjonam @stepankuzmin

[stepankuzmin]

Great! I think this is awesome!

[bewithjonam]

Looks fantastic. Thank you.

Right now,
throws error when you give a wrong clusterId (while retrieveing origin.x/origin.y)
throws error when you give a zoom level that doesn't have a tree. (not sure, should we use _limitZoom or throw a better error)

Fyi, trying to use this with a mapbox-gl-js fork

[mourner]

Thanks for the feedback! I should probably add some descriptive errors. Not sure about _limitZoom — if you change the input zoom, the cluster id relations will probably break.

[apkoponen]

Awesome, great work! Looking forward seeing this in Mapbox GL JS.