AABB constructor accepts erroneus arguments

<b>since ver 3.15</b>

The constructor ```AABB(int _x1, int _y1, int _x2, int _y2)``` needs to be modified to throw an error in the situations where _x2 <= _x1 or _y2 <= _y1. This means the AABB must be a rectangle in which the top left corner is always (x1,y1) and bottom right corner is always (x2,y2).