Still testing
Ajout des opérateurs

Vaut-il mieux faire un appel à 'copy' ou appeler un constructreur qui pourra
être redéfini ? Vaste débat ...
Exploitation des entités des niveaux pour améliorer leur couverture
Adaptation satisfaisante pour BreedableMob
Ajout de Creeper, Skeleton, Villager, Zombie

Le découpage entre villageois, villageois infecté et zombie est étonnant. À
se demander comment un villageois infecté, quand il redevient un simple
villageois, retrouve ses capacités originales
Cow, MushroomCow
CaveSpider, Enderman, Ghast, PigZombie, SilverFish, Slime and Spider
Removed insignificant classes

...but they may reappear later if nothing is found to distinguish a Spider
from a CaveSpider better than having different classes for them
Answer to previous commit:

For maintainability reason, even classes not adding any field from their
ancestor point-of-view disserve a class on their own. So that code
mentioning 'Spider' will always work. This also allows easy distinction
between a Spider and a CaveSpider for example
Following the same logic, Skeleton is different from a WitherSkeleton