Overhaul Observation Transformer API

[Skylion007]

🚀 Feature

• We are starting to use Observation Transformers as a general Habitat_Baselines tool and therefore it should be graduated one config level so it's no longer under the RL namespace.
• Decouple type for ObservationTransformer name, this allows multiple observation transformers of the same class with different args.

Better to do this sooner since more and more people are beginning to rely on ObservationTransformers.

[Skylion007]

Issue #500 might be a good time to do this general refactor.

[Skylion007]

Okay, so I think we may need to make the observation transfromer more like the Sensor API so that type and keyname are seperated. That way, a user can define multiple of the same observation_transformers affecting different keys just like they do currently with sensor types. ENABLED_SENSOR could then selected the obs_transformer based on their name, instead of their type:

    sensor = getattr(config.SIMULATOR, f"{sensor_type}_SENSOR")

Similar to this.