Add goal reaching as a terminated condition

[maxspahn]

It seems a good idea, #226, to trigger a terminated state once the goal is reached if a goal is present.

[behradkhadem]

I think goals are handled inside mpscenes and I don't have access to its code. But it's a good idea. It could have limited functionality (due to it being limited to position and velocity of things) but most RL projects in robotics require something put in some place, so I think we'll be fine!

[maxspahn]

@behradkhadem, do you think you could work on that?
By the way, mpscenes is public, see https://github.com/maxspahn/motion_planning_scenes

[behradkhadem]

I can but I don't know when I will have the free time to do that. It may be in a month or more.