Request for a few new standard environments

[zuoxingdong]

It would be nice to add a few more standard environments, such as following:

• Maze: spawn robot (Ant/Humanoid/Point mass) and the objective is to reach the goal
• Gather: spawn robot (Ant/Humanoid/Point mass) and the objective is to collect green dots and avoid red dots.

These also make people life much easier to quickly adapt similar environments.