add exception class for invalid initial conditions
add exception class for invalid initial conditions

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