Add Aabb2d::new and Aabb3d::new constructors

[Jondolf]

Objective

Currently, the only way to create an AABB is to specify its min and max coordinates. However, it's often more useful to use the center and half-size instead.

Solution

Add new constructors for Aabb2d and Aabb3d.

This:

let aabb = Aabb3d {
    min: center - half_size,
    max: center + half_size,
}

becomes this:

let aabb = Aabb3d::new(center, half_size);

I also made the usage of "half-extents" vs. "half-size" a bit more consistent.