changed background to use window height and width for calculating corePos
gravity of an obj is now calculated based on the size of its image. Current formula is area of image * 500,
which results in the same value for a 100x100 planet as before. Removed giant coefficient in gravitate() since
it is no longer necessary.
physical objects now have a self.pos vector containing self.x and self.y. have not yet updated all of physical
object to use this, but solarsystem will.