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/callbacks.jl

@@ -74,7 +74,7 @@ function callback_hrsc(state_u, state_t, env, P, isperiodic, hrsc::HRSC.Abstract
   @unpack mu, max_v   = get_cell_variables(cache)
   @unpack v           = get_static_variables(cache)
   @unpack rho         = get_dynamic_variables(cache)
-  dg1d.new_broadcast_volume!(maxspeed, equation, cache)
+  dg1d.new_broadcast_volume!(maxspeed, equation, mesh)
   Npts, K = layout(mesh)
   mat_max_v = reshape(view(v, :), (Npts, K))
   for k = 1:K