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.

% Run matlabs checkcode on all files in current folder including subfolders
function checkAllCode(d)
    default_arg('d','')
    files = findMfiles(d);
    checkcode(files)
end