dm_control robot base class compatiblity

[peterdavidfagan]

The base classes for robot arms used in dm_control are separated into robot hand and arm (see here).

I wish to open the discussion about aligning this repository with how robot arm entities are defined in dm_control by splitting the xml files into the hand and arm and attaching them via PyMJCF. Doing this may enable us to open a pull request on the dm_control repository to add the panda robot arm to the available manipulator entities.