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.

test/IntegrationTests/refs/fv_euler_isentropic_flow/fv_euler_isentropic_flow.toml

+[EquationOfState]
+idealgas_gamma = 3.0
+eos = "idealgas"
+
+[EulerEq]
+id = "smooth_isentropic_flow"
+
+[Evolution]
+cfl = 0.9
+tend = 0.50
+
+[Mesh]
+range = [-1.0, 1.0]
+k = 1048
+scheme = "FV"
+
+[Output]
+variables = ["rho", "q", "E"]
+aligned_ts = [ 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45 ]