Add three task scenes: move puck, tool use, bottle-cap unscrewing

[Manas-arumalla]

• move_puck_scene.xml — a low puck and two goal markers: push the puck
  to a goal with the closed gripper, or grasp and place it
  (pick-and-place).
• tool_scene.xml — a graspable stick and a block beyond bare reach:
  push the block onto a far goal using the stick.
• bottle_scene.xml — a bottle with a threaded cap: hinge + slide coupled
  like a screw (15 mm/turn), so turning the cap really lifts it off. The
  hinge is unlimited (hinge limits wrap past one revolution); the slide's
  range bounds the travel. Grasp welds for either arm.

Each scene loads standalone and holds its home keyframe cleanly (no
arm-object contact over an 8 s settle).

[k1000dai]

I realized that this scene can also be used as a pick-and-place task.
How about renaming the scene file name to move_puck_scene.xml and updating the task description?

[k1000dai]

@Manas-arumalla, thank you for adding more scenes! Here are some comments, so please have a look!

First, I feel that contact_scene.xml is a bit too simple and has no task goal. Maybe MuJoCo users can try it by themselves easily. I feel adding this scene is not necessary.

Second, puch_scene.xml seems to be useful for the pick and place task. As I commented above, could you rename the task name and description?

[Manas-arumalla]

1. Dropped contact_scene.xml. You're right that on its own it doesn't add much. It's useful with a controller driving it (which is how I use it), but as an isolated scene here, it carries no task, so I've removed it.
2. Renamed push_scene.xml to move_puck_scene.xml and broadened the description so it reads as a move / pick-and-place task rather than pushing specifically.

[k1000dai]

@kou Looks Good to me. Could you do a final check and merge?

[kou]

Could you update the PR title and description before we merge this?

[Manas-arumalla]

@kou I've updated them. Please review.

[kou]

Thanks.