preliminary version of clusters

Adresses issue #1573 . Preparation for clustering, with an example that clusters random points and prints the clusters to console. Things to be done:
1) Tests
2) Refactoring
3) Not clustering features outside the given extent
4) Recalculating clusters on zoom/extent change
5) Clicking on cluster should zoom to the extent of the clustered features
6) (maybe add support for more clustering algorithms?)

Any other suggestions are welcome.
this requires a typecast for the check-examples target `/** @type {Array.<number>} */ (size)` but be careful not to exceed 80 chars.