Pairwise sphere interactions for sphere-composite particles

[johnaparker]

If a single particle is a composition of spheres (like a dimer), then only the net force and net torque effect its dynamics. Stoked would treat this as a single particle. But for certain interactions (sphere electrostatic, collisions, van-der-walls, etc.), the interactions can still be treated more precisely at the sphere-sphere level.

Add a way to tell stoked that a particle is a composite of spheres, and then use that info to obtain the correct forces from sphere-sphere interactions.