Fix calculation of damping components.

I spent some time this afternoon looking at the forces on a trivial system and decided that the damping forces weren't being calculated correctly. The existing damping force calculation was based on relative velocities of the two beam endpoints. For example, a fictitious vertical damping force will appear when there is a difference in y velocities while the y positions are identical. This will cause a horizontal arm that is fixed at one end to not swing down.