Add Bhargava's decay function

Enhancement to Bhargava metabolic probe; introduced a property to enable/disable. Disabled by default, so there will be no effect on existing simulations.
I am OK with a post-hoc analysis that estimates the the crossings and uses those estimates to set the associated discrete state variable to pass the probe. True the answers would be different (even substantially different) especially if a crossing was estimated to occur when there was none or vice-versa, but it might outweigh re-running lengthy forward simulations.

The ClutchedPathSpring was easy because it uses a continuous state variable with very simple first-order dynamics.  The detection of the crossing (i.e. an event) is necessary in your case and a handler to set the discrete state variable from that point forward. @sherm1 does it makes sense to have an Event component? For the time being the probe can create and add an event handler to the system.