Cells now swim in the direction of the center of gravity of their spheres
Clicking will reset the position of the cells to their original coordinates
Spheres that are in a cell membrane will not attract spheres inside the cell
Revert "Spheres that are in a cell membrane will not attract spheres inside the cell"

This reverts commit 99718590995ce7bce4fc8c4375c49ba4502f0dbd.
Spheres that are in a cell membrane will not attract spheres inside the cell

This is so that spheres do not end up clustering inescapably at the
cell membrane.
Cells that leave one side of the window will return on the other
Disabled cell swimming