[FEATURE] merge entity

[Kashu7100]

You can combine two rigid entities in a scene (can be a mixture of URDF and MJCF!).
Use case: attach a gripper to an arm, attach a manipulator to a quadruped, etc.

Example use case (Panda + Figure hand):

[ziyanx02]

Well done! This will solve most of the problems! But there are some small issues here:

1. I have to change
   "genesis/assets/urdf/panda_bullet/panda_nohand.urdf" to "urdf/panda_bullet/panda_nohand.urdf"
"genesis/assets/urdf/panda_bullet/hand.urdf" to "urdf/panda_bullet/hand.urdf"
   to make the example work.
2. Once the link of the child entity assigned to merge is not the root link, changing its parent link will cause the child entity seperated into two parts. But this could be a quick fix for mobile manipulation stuff.

[Kashu7100]

Ok, fixed the pointed part.

1. fixed path to genesis assets
2. set default of child link to its base and when user assigns non-base link, it will warn in logger