Add animation demo

[tamakoshi2001]

tutorial for animation functions

[tamakoshi2001]

This is a tutorial notebook for #2203

[tamakoshi2001]

@nwlambert Thanks for your a lot of advice. I decided to focus on how to use the animation function with a simple example in animation-demo.md. I will use your advice when adding animation examples in other files.

[tamakoshi2001]

@nwlambert I add animations in other files. %matplotlib notebook changes the format of plot so I used a different way to show animations in them.

from matplotlib import rc
rc('animation', html='jshtml')

[tamakoshi2001]

This is a plot when using the magic code.

[tamakoshi2001]

@nwlambert Hi. Could you review the code?

[tamakoshi2001]

@Ericgig I ran pep8 check for all modified files.

[tamakoshi2001]

@nwlambert Thanks! I updated the notebook. The commit time is wrong because of my pc time. https://github.com/qutip/qutip-tutorials/pull/67/commits/bd2d2e75e8de717bf41990d7784cadd93f14aaa5