Introduce batch fk capabilities for floating and planar joints

[BuildingAtom]

Hello, it seemed like urchin's batched forwards kinematics functions didn't have support for planar and floating joints. I went through updated the relevant functions to add that support. To also simplify the forwards kinematics for those two joint types, I added functionality that would unpack a flat list or numpy array to the corresponding 2dof or 6dof inputs for those joints.

So, if you have a planar joint followed by a revolute joint, you can now do [x_val, y_val, rot_val] as your list input instead of [[x_val, y_val], rot_val], which helps with the numpy batching form for forward kinematics.

I also noticed that there were extra None type checks in get_child_pose and get_child_poses, so I also went ahead and removed those in one of those commits, but if that's a no-go, I can remove that commit.