Make plot_signals work when the signals have different signal shapes

The following fails because the Signal instances have the same navigation shape but not the same signal shape:

    s = signals.EELSSpectrum(np.random.random((10,20, 100)))
    im = signals.Image(np.random.random((10,20, 64, 64)))
    utils.plot.plot_signals([s, im])

This pull request tries to fix it.