Create a child node to parent

Property definitions

geometry $ SQuadTree :: with_parent
	# Create a child node to `parent`
	init with_parent(l: Int, c: Point[Numeric], w, h: Numeric, parent: QuadTree[E])
	do
		init(l, w, h)
		center = c
		self.parent_node = parent
	end
lib/geometry/quadtree.nit:247,2--253,4