Add Learning Agent to vacuum_world.ipynb

[apb7]

Hello, @norvig and @MrDupin! Currently, vacuum_world.ipynb discusses all the basic kinds of agent programs that embody the principles underlying almost all intelligent systems.
I am planning to add an explanation and a schematic diagram of the learning agent in vacuum_world.ipynb as well.
Please suggest.
Thanks!

[antmarakis]

I am not familiar with agents.py, does the learning agent have an implementation? I didn't find something. Also, can a learning agent be found in the book? If not, maybe you can implement the agent only in the notebook.

[apb7]

@MrDupin: Learning agent is given in the book as a general learning agent. It does not have an implementation in agents.py. I have added it to the notebook in PR #742.
Thanks!