Adding registration/RMS error for cartesian poses

[adeguet1]

Most tracking systems (optical, magnetic...) provide some kind of information to help determine how reliable the pose estimation is. For example, Atracsys and NDi provide a single floating point RMS error. The meaning and scale of these numbers is likely vendor/implementation dependent so it's hard to standardize them. As usual, the two questions are what to provide and how:

• What:
  • At least the raw number provided by the device
  • Maybe a reference number to allow the user to determine what is good or bad. Alternatively, we could add an enum with predefined values: good, marginal, bad, not_found
• How:
  • Add an extra topic with a simple message. See for example Sending per-tool registration error to ROS jhu-saw/sawAtracsysFusionTrack#7
  • Add a new and more powerful topic measured_cs for cartesian state and have a payload that contains the pose as well as the RMS. See also Extending servo commands #6.