WIP Test/nonisothermal
See !2473 (closed). Based on !2471 (merged)
Shows unphysical temperature gradient. Edit: This is expected. Even for an incompressible fluid viscous dissipation leads to small temperature changes. However, this term is not correctly implemented (see #1256)

See if we can get a better solution by adding \vec{v}\cdot\nabla p
in the energy balance somehow Edit: adding this term leads to ignoring viscous dissipation. But does not correct the original mistake in the implementation (see #1256)
For incompressible fluids we have at least two options:
* Assemble internal energy fluxes instead of enthalpy fluxes, the volume work term disappears (not generic)
* Add the missing term and keep assembling enthalpy fluxes
The solution is to include the gravity contribution (see #1256)