Bug fix, slight optimization, and new test case

Fixed an egregious bug in the original GameboardLW class that I uploaded
earlier. Also made an optimization to the OptimalRobotsPlayer algorithm
to further reduce space required. The easy_test_board image offers a
better test case to use. OptimalRobotsPlayer finds the solution,
involving 3 robots and 5 moves, after 41 seconds on my machine.