Update examples to generate animated plots

[wheelerMT]

Feature Request

Rather than static plots showing one frame of data, why not make use of all the frames of data generated in the examples and plot a movie of the evolution. This showcases the capabilities of the library better, and is overall more interesting.

Solution

The easiest way to do this is using Matplotlib's [FuncAnimation}(https://matplotlib.org/stable/api/_as_gen/matplotlib.animation.FuncAnimation.html) class, meaning we can rely on dependencies already specified for the library. To do the animation, we can create an imshow of, e.g., the atom density at each frame, and stitch the result into a .gif file. The initial size of the grids may have to be lowered to, e.g., 256 x 256 if the animation takes too long to render.