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/utils.jl

@@ -270,7 +270,7 @@ end
 
 
 TODO() = error("Not implemented yet!")
-TODO(msg::String) = error("Not implemented: $msg")
+TODO(msg::Union{AbstractString,Symbol}) = error("Not implemented: $msg")
 TODO(fn::Function) = error("'$fn': Not implemented yet!")
 TODO(fn::Function, msg) = error("""
 '$fn'' Not implemented yet!