Improved script for ex. 4.7

[CorentinJ]

Hello, I have submitted my code for the exercise 4.7, in place of the current one.

• It is optimized using vectorization and memoization. It takes under a minute to compute the optimal policy.
• The code is configurable through global constants
• The additional constraints from the exercise 4.7 have been implemented
• The code produces a pretty plot of the policy and of the value function

I also didn't get results exactly equal to that of the book, but they're very close.

Plot of the base problem as presented in example 4.2:

After introducing the two nonlinearities: