Initialize self.jac_fn even if analytic jacobian is not used
Initialize self.jac_fn even if analytic jacobian is not used

Thanks to @JamesPino for the original fix.

jmuhlich/pysb,LoLab-VU/pysb,johnbachman/pysb,johnbachman/pysb,LoLab-VU/pysb,alubbock/pysb,pysb/pysb