Bugfix stack axis

The ``stack`` function generates a bug:

    s = [signals.Spectrum(range(i,10+i)) for i in range(4)]
    s1=utils.stack(s)
    s1.axes_manager.signal_axes[0].scale = 0.005
    s1.plot()

The signal scale of the plot is not updated.

I fixed ``stack``, but it might be a problem coming from deeper.