Create discrete action collaborative mission

[DaveyBiggers]

For example, resource collecting.

[DaveyBiggers]

Alternative example: "catch"? Player A tries to reach player B.
If it succeeds, +ve reward for A, -ve reward for B.
If mission times out (number of steps exceeded): +ve reward for B, -ve reward for A.

Will require new reward handler - RewardForRelativePosition?

[DaveyBiggers]

See the malmo-collaborative challenge (https://github.com/Microsoft/malmo-challenge)