Gym environments

Upkie has environments compatible with the Gymnasium API:

• UpkieGroundVelocity: behave like a wheeled inverted pendulum.
• UpkieServoPositions: joint position control.
• UpkieServoTorques: joint torque control.
• UpkieServos: control joint servos directly, including feedforward torques, position and velocity feedback.

While each environment has its own observation and action spaces, all of them also report full spine observations in their info dictionaries.