Add RK6 with residual viscosity update and reduce computational effort of spatial scheme - Add RK6 with residual updates - Change the D2 operator for upwind schemes to one less computationally expensive.

function q = convergence(e,h)
    for i = 1:length(e)-1
        q(i) = log(e(i)/e(i+1))/log(h(i+1)/h(i));
    end
end