Add assertion for maze constraints and limit _random_floor_cell attempts

[Gjorgji-Noveski]

During maze generation in game environments, an issue can occur if the min_grid_size -2 is not >= min_dist. This can cause an invalid maze layout to be generated in which there is no place to put starting cell and goal cell on the grid. This happens because the outer borders of the grid are used as walls to constrain the agent.

Given the following maze configuration:
size=6, min_dist=2, max_dist=10, min_grid_size=3, max_grid_size=12, seed=1
the following maze would be generated before placing any (Start) and (Goal) cells:

Under these conditions, it is not possible to place the start and goal cells.

Additionally there is no guard against infinite loop in the _random_floor_cell function, since it uses a while True loop.

This PR adds an additional assertion check in maze.py and additionally limits the number of tries the
_random_floor_cell function can perform before raising a RuntimeError

Steps to reproduce:

import reasoning_gym

data = reasoning_gym.create_dataset("maze", size=6, min_dist=2,
                                    max_dist=10, min_grid_size=3,
                                    max_grid_size=12, seed=1)

print(list(data))

The generation would stall and run indefinitely