EulerEq: Rework Navier-Stokes regularization

The goal is to reproduce plots for the shock tube tests from the Zingan+ 2013 paper.

We rework the Navier-Stokes regularization to work more like in the Zingan+ 2013 paper. Before we tried to mimic a Guermond+ 2014 paper, but they did not provide any shock tube tests.

This PR adds new parameters to the EulerEq section:

• av_navierstoke_prho, av_navierstoke_pt: these are effective Prandtl numbers that can be used to control how much of the viscosity coefficient for the momentum regularization should be added to the density and energy regularization.
• id_smooth: If true smooth initial data profile with a Bernstein filter
• av_drag: Dragging factor to throttle AV evolution
• av_recompute_substeps: If true AV computation is scheduled to run on each substep.

We also fixed a sever issue in one of the formulae. In the computation of maxspeed we accidentally compute q/p instead of q/rho as one part of the wave speed. Fixing this seems to have a positive influence. I am curious how this will effect the ref tests. Answer: I had to update all associated reftests, but all changes were minor.