Fix deprecated module and attribute errors in visualization and plot utils

Replace the deprecated scipy.ndimage.imread module with matplotlib.pyplot.imread in visualization/visualize_frame.py to fix the AttributeError

Also, update the utils/plot_utils.py file by replacing the attribute error caused by the DiscreteSlider object with self._observers.process('changed', discrete_val)

These changes have been tested on macOS 12.6 with matplotlib version 3.6.2.

thanks 👍