Move position vector from planet to quadtree

Move position vector from planet to quadtree

Moving the position information inside the quadtree structure will
make it eventually much easier to do varios tree operations within the
quadtree code. Most of the quadtree callback functions can be moved
inside the quadtree code. This patch implements just the first part of
the change, moving the coordinates inside quadtree.

Signed-off-by: Timo Kokkonen <kaapeli@itanic.dy.fi>