Add method to move entity from one entity manager to another

[hak33m16]

I'm using the entity manager class for all entities that are currently loaded into the game, as it has many useful features for querying entities with specific components. However, I often find myself having to create entity vectors in order to know what entities I should be querying based on where somebody currently is in the game, which obviously ends up leading to me re-creating querying this structure. I think there would be a lot of use for this if it were added in