Add deleteInRange, clear, filter

[crhallberg]

This work updates #42 (with many thanks to @AndreAzevedoCoder) with bits from #12 (many thanks to @edenkl8 and @Dezzles)!

• deleteInRange(range) - delete all points inside of range.
• filter(fn) - works like Array.filter(): returns a new QuadTree with only the points that return true.
• clear() - removes all points and children from a QuadTree.

Closes #9, #41.