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. This leads, for example, to a fictitious vertical damping force 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. I believe an improved implementation calculates the stretch rate of the beam and then the x/y components of damping force based on the beam direction.