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 never claimed that an Analysis adds Probes. I was referring to the use case where CMC is run and the resulting states file is then passed to the Analyze Tool after the user adds metabolic probes to the model. We have been advertising that it is valid to do this provided `probe_operation != "integrate"`; however, it seems to me that we actually need a witness function to detect 0.1-crossings and the integrator needs to localize these crossings in time. Should I be following [the implementation of ClutchedPathSpring](https://github.com/opensim-org/opensim-core/blob/master/OpenSim/Actuators/ClutchedPathSpring.cpp#L166)? Note that this will have the undesirable property of changing the results when a probe is added (though Heisenberg tells us this is okay). We also need to give an error message if the user attempts to use the Analyze Tool to run the Bhargava probe with Bhargava's decay function (but not in other cases).