Fix bug in SAT terms for upwind operators - Set the appropriate boundary derivatives in respect to choice of upwind discretization of D2 - Distinguish between upwind discretizations e.g upwind+ vs upwind-

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