New collection to store edible organisms, Methods changed.

1) HashSet<Organism> canEat: changed from storing String objects of the organism's ID to actually storing Organism objects, so they can be compared to the organisms living in a square in the future. Allows more options for comparison and authentication of food.

2) New Methods added to change the current health of an organisms. Increade/Decrease by one for organisms with maxHealthPoints > -1. kill() method for those who needs to "die" instantly or for plants.
Note: In case an organism dies (currentHealthPoints == 0 && isAlive == false), its currentHealthPoints can no longer be changed (increase/decreased.)