add option for alternative heuristics

[mivanit]

currently we use manhattan distance as the heuristic:

def heuristic(a: CoordTup, b: CoordTup) -> float:
    """return manhattan distance between two points"""
    return np.abs(a[0] - b[0]) + np.abs(a[1] - b[1])

(see maze_dataset/maze/lattice_maze.py)

This works fine for acyclic mazes, but is non-deterministic for some cyclic ones (this is particularly visible for high-p percolation mazes).

Ideally, it would be nice to be able to specify a solution heuristic in the MazeDatasetConfig. This would require modifying a number of functions to accept such a parameter:

• LatticeMaze.generate_random_path()
• get_maze_with_solution()
• _generate_maze_helper()
• SolvedMaze.from_targeted_lattice_maze()