Delete agents by agent id

[aaravpandya]

This PR introduces feature to delete agents by their agent id.

To use this feature, you can call sim.deleteAgents(world_to_agent_dict). The dict structure is {world_idx: [agent_ids]}. Sample code:

sim.deleteAgents({0: [1408]}) # delete agent id 1408 in world idx 0

We set an array deletedAgents with agent ids. We reuse functionality of setMaps where we destroy all agents and reinitialize them. But we ignore the agents specified in deletedAgents. So the agents are not initialized. Remember, because of this the indexing will change. For eg an agent at idx 5 will be at idx 4 after an agent is deleted.

Also, the deletedAgents array is permanent. You can reset this tensor with -1 values to reinitialize all the agents again. However, I reset the deletedAgents array when you call setMaps to change the map on runtime because the agent ids will have changed.