Function to trim dead chunks off board

[emilyyjia]

The board in the game of life is infinite. Ideally we want to store the portion of the board that contains the alive cells. Given how the game of life progresses, we could end up with a big chunk of the stored board all dead, therefore we might want to implement a function that trims off this section

[emilyyjia]

Here's my best attempt at explaining in a drawing. So all the cells outside of the blue highlight should be removed from the board

[emilyyjia]

not exactly like the above picture, but all dead cells are trimmed off every iteration