Merge: quadtree: use an array, fix warnings and standardize the doc

Merge: quadtree: use an array, fix warnings and standardize the doc

Replace the 4 attributes by an array of all the children. This new structure lightens the code and allows to fix 65 `method call on a nullable receiver` warnings. As this is already a significant refactor, I avoided modifying further the implementations to avoid breaking things.

Pull-Request: #2528