experimental hair tuning

Ryan and I made a few tuning modifications to the experimental verlet hair feature.
BTW, the hair simulation is very sensitive to the value of deltaTime.  When the fps varies it will cause stretching and shrinking of the hair strands -- looks like twitching.  This can be fixed/reduced by either using a constant deltaTime here, by looping over the constraint enforcement (below) multiple times, or by a more rigorous antisymmetric enforcement of the constraints (slave child to parent along the chain, accumulating corrections for "lastPosition" as we go).