small change to subsystemsMakeHist

There actually is no change to runFalcon. I just fixed the inputs of convertIrrevFluxDistribution to include the model because my previous version did not have that. 

subsystemsMakeHist works the same, but I added one additional input. The includeZero input specifies whether or not zero intensity values should be counted. In the previous version they were automatically counted, now you can specify if you want to count them or not. I am running it now to see if the histograms turn out similar or if they are very different