Improve MultiSphereShape rendering

[jslee02]

Previously, DART only draws the spheres in a MultiSphereShape while the actual shape is the convex hull enclosing all the spheres. This PR still doesn't render the convex hull but improves the visualization by drawing the cylinders that connect the pairs of the spheres.

Before:

After:

[mxgrey]

I'm a bit concerned by the name of the class. I don't think the name really conveys what the class is actually representing. This whole time, I've been thinking that the MultiSphereShape is something more like a point cloud. What if it were called something like MultiSphereConvexHullShape? Or if there's a technical term for this type of shape, that would probably be better. We could keep a MultiSphereShape alias for backwards compatibility. I can open this as a separate issue if you think it warrants more of a discussion.

[jslee02]

I agree with you. I named it inspired by Bullet's btMultiSphereShape and commented that it's a convex hull shape, but it would be much better to change the name to show that explicitly.

MultiSphereConvexHullShape sounds good to me. I also came up with SphereSweptConvexHullShape, but it could confuse people that the shape was created by sweeping with a single sphere I think.

By the way, let's address the naming in a separate PR to wait for other feedback on the name.

[psigen]

Sounds reasonable to me. It might be enough to name it `MultiSphereHullShape` too. But adding some "hull" terminology seems important.