support for multi-agent damain

[nina124]

The introduction of gym-soccer says

The Soccer environment is a multiagent domain featuring continuous state and action spaces.

However, currently the gym-soccer only supports:
offense_agents=1,
defense_agents=0,
offense_npcs=0,
defense_npcs=0

Could it be updated that we can specify the number of offense_agents, defense_agents, offense_npcs and defense_npcs? Also, in this case, we can get the states of all the agents at the same time.

[YupengGao]

@nina124 Do you currently have any idea how to make it support multi-agent ? any thoughts on this?