Ft multi agent support

[c-salmi]

To support the centralized control of multiple robots through a single mppi, it was necessary to change quite a few thing in the structure of the IsaacGymWrapper.

• Every robot (or other actor) has it's own config file that matches the ActorWrapper dataclass.
• A list of these actors can be specified (by their file name) in the ExampleConfig. These are then automatically loaded in the IsaacGymWrapper. If the actor has a type robot it will be considered to be controlled by the mppi.
• Check the multi-jackal.py or the multi-point_robot.py examples to see this in action.
• The handling of the differential drive ik is moved from mppi_isaac.py to the IsaacGymWrapper.
• Caster joints can be specified in the actor config file and their friction is automatically set to zero.

[cpezzato]

Nice!

I have checked most of the examples but it looks like the boxer in boxer.py does not move in pybullet due to friction of the caster wheels. The samples are fine though in isaacgym and they can move

[c-salmi]

There was a bug in the wrapper that caused the robot positions in the cost function for the boxer example to be zero. I don't think the friction of the caster joint was the issue, but it should be fixed now.