2D TransForm and 2D rigid body

[FeiZhu]

It seems that Transform does not support 2D transformation. We need 2D transformation. @Vovery

SurfaceMesh is 3D. Rigid body is SurfaceMesh based, hence it seems that we cannot simulate 2D rigid bodies. We need 2D rigid body support. @TianxiangZhang

We could define a 2D counterpart of SurfaceMesh, Polygon maybe? For 2D rigid body, it contains a Polygon. BTW, does BVH support 2D?

Let's find an elegant solution.

Whatever simulation we do, we want both 2D and 3D.

[FeiZhu]

If we cannot integrate 3D&&2D rigid body into one class, perhaps we should name them RigidBody2D and RigidBody3D. Nevertheless, we should consider integrate 2D/3D into one RigidBody class first.

[FeiZhu]

The interface of Transform has been changed to adapt 2D transformations, please modify your code involved. You need to replace your original use of Transform with Transform<Scalar,3> as you'll use it for 3D transformation.

@Vovery , I revised your commit, namely: reorganized transform architecture, include necessary header file, remove bad coding behavior. Please review it and make sure I didn't do anything bad.