Add a `packed_spheres` function to unify gravity and electrostatic packings

[jgostick]

Both the gravity and electrostatic packing work by placing the spheres to be inserted as close to an 'attractor' as possible. In the case of gravity, it's the bottom edge of the image, in the case of electrostatic it's the user-specified seed points. These could clearly be unified into a single function since the both do the same thing. To be consistent with the other functions this should be called packed_spheres (to match random_spheres and lattice_spheres).