Add Flexible Joint States message design doc

[bmagyar]

Draft for flexible joint states message

[bmagyar]

@jordan-palacios @Karsten1987 I've tried to collect my current ideas and a general specification of what I'd like to see out of this. Please scrutinize and suggest changes, this is more of a discussion opener than a strict design doc at the moment :)

[gavanderhoorn]

Reminded me of ros-controls/ros_controllers#416 (there are probably others).

[bmagyar]

Thanks for that ref @gavanderhoorn ! The end of the unholy trinity of pos-vel-eff is drawing nigh!

[gavanderhoorn]

Would it make sense to try and get some insight into how other control frameworks are modelling / encoding this type of data?

Might provide valuable insights.

What about going for the channel approach (ie: give each value-type its own topic)? JointState is right now typically used as a bus. I appreciate the added complexity of having N topics for N value-types, but it might be worth it.

Another alternative: use a semantically meaningless float64 for everything, and overlay semantics by publishing/providing an Info message. This would be somewhat similar to CameraInfo, where (additional) information is provided OoB. This would not be too nice from a system modelling perspective, but would provide lots of flexibility. Channels between controllers would then exchange primitive data-types instead of composites.

[bmagyar]

This document is by no means done, please feel free to open further PR-s with more proposals I am however going to merge it for visibility.