New constructors for Rectangle, Box2D, Box3D, and possibly others

[nathan-alden-sr]

A few handy constructors could be added to several types. I noticed this when using Rectangle, but it looks like other types like Box2D and Box3D could benefit. These constructors would allow one to pass in each discrete component value rather than having to construct vectors. This helps readability in user code.

For example:

public Rectangle(T originX, T originY, T sizeX, T sizeY)
    : this(new Vector2D<T>(originX, originY), new Vector2D<T>(sizeX, sizeY))
{
}

I'll look around at the various classes and see if I can knock them all out.

[nathan-alden-sr]

Merged #657.