[ODE] Enabling strategy

[Morgan McGuire]

I use ODE's auto-disabling to let bodies with zero velocity and zero net
external force "sleep."  I've run into two problems:

1. Bodies with lots of joints (say, four) don't sleep.  I think this is because
micro collisions keep blinking in and out.  Since waking up a body wakes up
everything attached to it, an object with N parts is N times less likely to
sleep.  I manually set tiny velocities to zero but can't get around the
collision issue. 

2. Auto-wake up can't handle situations where there is no new collision.  Say
you have a stack of boxes that is sleeping.  If the bottom box is removed, or
carefully slid sideways, then it never collides with the boxes above it.  That
means that the rest of the stack will hang in the air until something hits it
to wake it up.

Any wisdom on these?

-m