Extend joint state interface by commanded position

[martiniil]

It would be helpful if a controller that does not intend to claim a resource could not only read the actual position but also the commanded position. Something like

  double getCommandedPosition()  const {assert(cmd_pos_); return *cmd_pos_;}

I see that the interfaces are not set in stone (#395). Looking forward to get some feedback on that before I do the actual work :)

[bmagyar]

Now is the time, impress us! :)

What I have been trying to achieve lately is to handle all interface types (known and unknown (!)) in a flexible way so perhaps a way to query all commanded aspects of a joint would even be better. I'm not sure what'd be the best way of doing it as the holy trinity of position-velocity-effort is pretty much hardcoded everywhere. See #392 as my current attempt (doesn't even compile, let alone is correct) that does something along those lines.