Add Compound Sensors

[DavidPL1]

This PR, based on the discussion in #26, overhauls the MuJoCo sensor to ROS serialization and introduces compound sensors.

The currently available compound sensors are Twist, IMU, Wrench, and Pose. New ones can easily be added.

Points to Discuss

• Is a factory for compound sensors overkill or worth the reduced maintenance cost?
• Sensors used in compound sensors are not separately published
• Should the frame ids of sensors used in a compound sensor always be the same?
• Defining compound sensors requires a list of
  • sensors
  • (frame_id? For now we assume the frame id of the included sensors which should be coherent)
  • frequency
  • type
  • name
  • publishing rate (at some point when the feature is introduced) Since the custom tag does not really allow adding all this information conveniently, I moved the whole configuration to the plugin configuration.

TODOs

• [ ] IMU covariance configuration is NYI
• [ ] Tests for compound sensors