Adding radiative energy transport

On last December with @fmasset we went through the derivation of the energy equations quite in detail.

Our conclusion is that the way we treat internal and radiative energy is ok a part one point to be tested.

Up to now we have neglected the advection of the radiative energy.

However, this term appears in the orginal derivation of the equation for the temporal evolution of the radiative energy by Mihalas and Mihalas (Foundations of Radiation Hydrodynamics) and its importance is stated in the paper "Monte-Carlo Radiation Hydrodinamics with implicit methods" (Roth, Kasen 2014). In this paper it is shown that even for fluid speed v<<<<c radiation advection is required to correctly transport the energy.