[Feature Request] Improve API consistency

[bheijden]

🚀 Feature

• Rename actions/observations to outputs/inputs, respectively in the Graph.
• Only refer to outputs/inputs as actions/observations if GymBaseEnv is used. Else stick to outputs/inputs naming convention.
• Rename sensors/actuators to outputs/inputs, respectively in the EngineGraph.

Motivation

Even though the API in Graph and EngineGraph for nodes (inputs/outputs), objects (actuators/sensors), and the environment (actions/observations) are practically the same, they have different names. This unnecessarily confuses new users. Referring to all of them as either an input or output will make the API more consistent and easier to grasp.

Checklist

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