Gridworld environment for reinforcement learning from Sutton & Barto (2017).
Grid of shape 4x12 with a goal state in the bottom right of the grid.
Episodes start in the lower left state. Possible actions include going left, right, up and down.
Some states in the lower part of the grid are a cliff,
so taking a step into this cliff will yield a high negative reward of - 100 and move the agent
back to the starting state.
Elsewise rewards are - 1, for the goal state 0.