[WiP] Reproducible on- and off-policy sampling

[MkuuWaUjinga]

Extend the Environment API to support setting environment library specific seeds.

Tasks:

• Extend Environment interface
• Set seeds for Gym envs
• Ensure seeds are set when Sampler classes start working
• Set seeds for dm_control envs
• Set seeds for grid_world envs
• Set seeds for point envs
• Set seeds for metaworld envs

Open Questions:

• How to ensure determinism of off-policy algorithms?

[MkuuWaUjinga]

Thanks for the pointers. Addressed everything in the latest commits. I assume GridWorld and PointEnv don't have any seeds at all?
Furthermore, with the implementation right now every worker has the same environment seed. This means that each worker always samples the same trajectory given a fixed action sequence. I think this is something we need to fix before merging?