Added a default constructor and the zero vector.
Added a default constructor.
Initialized the private members in the constructor.
Fixed a ton of bugs related to when exceptions were being thrown.
Refactored the event calling loops into a couple private methods.
Added some new event types related to entities.
Totally refactored this to be more structured. It still isn't finished though...
Set it so that entities can only be added once to a simulation, and to only one simulation at a time.