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/euler_isentropic_flow_regulate-ns/euler_isentropic_flow_regulate-ns.toml

+[EulerEq]
+id = "smooth_isentropic_flow"
+av_regularization = "navierstokes"
+# av_regularization = "mono"
+
+[EquationOfState]
+eos = "idealgas"
+idealgas_gamma = 3.0
+
+[Mesh]
+range = [ -1.0, 1.0 ]
+cfl = 0.10
+n = 7
+k = 201
+basis = "lgl"
+
+[Output]
+every_iteration = 1
+# every_dt        = 0.02
+variables       = [ "rho", "q", "E", "smoothed_mu" ]
+
+[Evolution]
+tend = 0.5
+
+[Log]
+progress_stdout = true
+
+[HRSC]
+method = "av"
+av_smoother_order = 2
+av_method = "mda"
+# av_method = "entropy"
+# entropy_cmax = 2.0
+# entropy_ce = 1.0
+# mda_cmax = 1.0