Feature: Non-signaling value for XOR nonlocal game

[vprusso]

Presently, the nonlocal_games/xor_game.py file has functions classical_value and quantum_value that are responsible for computing the classical and quantum value of a nonlocal game, respectively.

This task would include the ability to also compute the non-signaling value of an XOR nonlocal game. This value serves as a natural upper bound to both the classical and quantum values.

This would be included in the xor_game.py file with the function name nonsignaling_value. Proper tests would also need to be in place as well for known non-signaling values of given XOR games. For instance, it is expected that the non-signaling value of the CHSH game is equal to 1.