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.
Since probes can be added after-the-fact in an Analysis, it isn't obvious to me how we can ensure the Bhargava probe always reports the same values. Any suggestions on how to fix this implementation, or do we need to pursue a better design for the metabolic calculators? :turtle: