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.
Probably a good idea. We could also annotate it with "experimental" or something like that (it is disabled by default, after all). Ideally, the output would _not_ depend on the frequency with which the reporter is called, but that would require the probe to be more than a strict output (we would need to isolate 0.1-crossings in time). I'll add a note. And a cow :cow: