[WIP] Add Aabb2

This adds an Aabb2 class to the library.

Currently it has no transform and rotate methods because I am not quite sure what exactly the difference between absoluteRotate and transform are and why the Aabb3 class takes an Matrix4 instead of Matrix3.

But the Aabb2 class has the following three methods, which could also be added to the Aabb3 class:
void setFromCombination(Aabb2 a, Aabb2 b)
bool contains(Aabb2 other)
bool overlaps(Aabb2 other)

Maybe intersectsWith would be a better name instead of overlaps.