Pr move distractions closer in curriculum

[le-horizon]

This includes several changes to allow adjusting goal's direction in curriculum, allow resetting and picking new random goal in one episode, moving distraction objects also according to curriculum, using polar coordinate in randomly setting goal position.

[Jialn]

As of inserting more than one object of the same model to world, there is a function of insert_model() in gazebo_base.py can be used, supporting both specified name and position.

    def insert_model(self, model, name=None, pose="0 0 0 0 0 0"):
        """
        Insert a model with a name into a specific position of the world
        Args:
            model (string): the name of the model in the model database
            name (string): the name of the model in the world. 
                     If not provided, it's the same as the model name.
            pose (string): the pose of the model, format is "x y z roll pitch yaw"
        """

[le-horizon]

@emailweixu I rebased the change on new master, could you take a quick look?

Thanks, Le