Implement regularizations for Euler eq

Implements the Navier-Stokes and general regularization from J.-L. Guermond and B. Popov, “Viscous regularization of the euler equations and entropy principles,” SIAM Journal on Applied Mathematics, vol. 74, no. 2, pp. 284–305, 2014.

src/EulerEq/EulerEq.jl

@@ -49,6 +49,22 @@ dg1d.@parameters EulerEq begin
   bc = "periodic"
   @check bc in [ "periodic", "from_id" ]
 
+  """
+  Type of artificial viscosity regularization. Available options:
+  - `none`
+  - `mono` - monolithic regularization, see [1]
+  - `navierstokes` - regularization based on Navier-Stokes equations, but without heat flux, see [1]
+  - `general` - most general regularization, see [1]; actually implemented version of [2]
+
+  [1] J.-L. Guermond and B. Popov, “Viscous regularization of the euler equations and entropy principles,”
+  SIAM Journal on Applied Mathematics, vol. 74, no. 2, pp. 284–305, 2014.
+  [2] J.-L. Guermond, B. Popov, and V. Tomov, “Entropy–viscosity method for the single material euler equa-
+  tions in lagrangian frame,” Computer Methods in Applied Mechanics and Engineering, vol. 300, pp. 402–
+  426, 2016.
+  """
+  av_regularization = "none"
+  @check av_regularization in ["none", "mono", "navierstokes", "general"]
+
 end
 
 end # module