This function provides a way to subtract off the quantity depleted from a finite source during a time step. It is used in conjunction with constInjectionRate. See constInjectionRate for an example.
Parameters: C r 
C is the field containing the remaining source quantity
r is a variable containing the rate of injection
should be the TIME_STEP variable
The actual rate depleted depends on rate, the quantity remaining, and the time step. If the quantity remaining is non-zero then the rate of injection is the minimum of the maximum constant rate of injection and the rate to inject the remaining source quantity over the time step.