Implement Navier-Stokes regularizations for Euler eq

Continuation of !71 (closed)

Implements the Navier-Stokes 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.

PR also contains some other fixes:

• refactor logging details of IntegrationTests; in particular, we now report the Inf norm of differences together with the grid point where the comparison failed
• add a 0th order option for SmoothedArtificialViscosity
• added examples for sod shock tube problem and isentropic flow

examples/euler_isentropic_flow/mono.toml

+[EquationOfState]
+idealgas_gamma = 3.0
+eos = "idealgas"
+
+[EulerEq]
+id = "smooth_isentropic_flow"
+av_regularization = "mono"
+
+[Evolution]
+cfl = 0.75
+tend = 0.50
+
+[Mesh]
+range = [-1.0, 1.0]
+n = 5
+k = 680
+basis = "lgl"
+
+[Output]
+variables = ["rho", "q", "E"]
+aligned_ts = "$(collect(range(0.01,0.50,step=0.01)))"
+
+[HRSC]
+method = "av"
+av_method = "entropy"
+entropy_ce = 20.0