add examples mono and navier-stokes regularization of euler_isentropic_flow problem

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

examples/euler_isentropic_flow/navier.toml

+[EquationOfState]
+idealgas_gamma = 3.0
+eos = "idealgas"
+
+[EulerEq]
+id = "smooth_isentropic_flow"
+av_regularization = "navierstokes"
+
+[Evolution]
+cfl = 0.05
+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