Before revision:  Phys., 328, 2017 , solution positivity is only numerically enforced. In this paper we design a novel DG method, which with Euler forward time discretization is shown to preserve  positivity of cell averages at all time steps.
After revision:  Phys., 328, 2017 Our DG method with Euler forward time discretization is shown to preserve positivity of cell averages at all time steps.