Removing calls to allow substrate or product to be 'None'
Removing calls to allow substrate or product to be 'None'

neurord/pysb,LoLab-VU/pysb,johnbachman/pysb,neurord/pysb,jmuhlich/pysb,pysb/pysb,neurord/pysb,LoLab-VU/pysb,d-fan/pysb,spgarbet/pysb,johnbachman/pysb,spgarbet/pysb,alubbock/pysb,d-fan/pysb,spgarbet/pysb,d-fan/pysb