constructor from an ordered pair of PoolName (string like) objects and a function with the set of formal argument names forming a subset of the state_variable_names
# S4 method for `function`,character,character,missing
InternalFlux_by_PoolName(func, sourceName, destinationName)
A real valued function describing the flux (mass/time) as function of (some of ) the state variables and time.
A string identifying the source pool of the flux
A string identifying the destination pool of the flux