Minigrid: Submit MultiEnv Environment PR so we don't need to maintain this code in DTI.

[jbloomAus]

I wrote a class that mixes environments/hyperparameters called a Multienv which is kinda useful for training, however it should probably live in the Minigrid github repo.

If someone has the time to submit that as a PR and write whatever tests their maintainers require (probably not a huge amount, then this would be greatly appreciated.

Checklist: [ ] make a PR to Minigrid with the Multienv [ ] add tests to the PR [ ] have the PR accepted [ ] when the PR is accepted, update this repo to use the new minigrid version and remove the multienv code from this repo.