【强化学习】解决MPE环境中两个小球重合导致态势为nan问题

本质原因还没定位到，具体为某种操作，导致两个具有碰撞属性的小球重合了！！！

重合后导致距离计算为0，从而导致碰撞的力为nan，导致态势位置变成nan。

修改以下源码解决：
multiagent/core.py中的193行改为：

force = self.contact_force * delta_pos / (dist+1e-6) * penetration

完整的 get_collision_force 方法：

    # get collision forces for any contact between two entitiesdef get_collision_force(self, entity_a, entity_b):if (not entity_a.collide) or (not entity_b.collide):return [None, None] # not a colliderif (entity_a is entity_b):return [None, None] # don't collide against itself# compute actual distance between entitiesdelta_pos = entity_a.state.p_pos - entity_b.state.p_posdist = np.sqrt(np.sum(np.square(delta_pos)))# minimum allowable distancedist_min = entity_a.size + entity_b.size# softmax penetrationk = self.contact_marginpenetration = np.logaddexp(0, -(dist - dist_min)/k)*kforce = self.contact_force * delta_pos / (dist+1e-6) * penetrationforce_a = +force if entity_a.movable else Noneforce_b = -force if entity_b.movable else Nonereturn [force_a, force_b]