Add support for artificial viscosity - Add option to use upwind operators for discretizing the scheme - Add option to use dissipation operators constructed from undivided differences when discretizing with narrow stencil operators

function A = spdiag(a,i)
    default_arg('i',0);

    if isrow(a)
        a = a';
    end

    n = length(a)+abs(i);
    A = spdiags(a,i,n,n);
end