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.
Does the ProbeReporter (Analysis) add Probes? I don't think so. Probes are added to the Model first. They are ModelComponents and therefore have legit internal states. Did you test the case where you get a control > 0.1 due to a trial integration step (e.g. due to feedback error) and the state is backed off? When called again does the state have the previous stimStatetime?

In the Reporter (Analysis) you have to make sure to set the stimtime state prior to calling computePropeInputs, but otherwise it should work in both cases.