Add viscous terms of Guermond and Popov for regularization, probably using BR1 with entropy variable gradients to get ES

[ranocha]

In GitLab by @gregorgassner on May 4, 2020, 10:28

Viscous Terms can be found in https://arxiv.org/abs/1212.5566

Explicit form and numerical tests are here

http://user.it.uu.se/~nazar000/publications/preprints/Nazarov_Larcher.pdf

[jlchan]

Explicit formulas in terms of entropy variables can be found in https://arxiv.org/pdf/2109.06653.pdf