[Rock-dev] Representation of invalid positions vs. unknown uncertainty

[Sylvain Joyeux]

While debugging some things for iMoby, I stumbled across the fact that 
iMoby's odometry data has a position covariance matrix set to

   Inf NaN NaN
   NaN Inf NaN
   NaN NaN Inf

Which is the default value set by RigidBodyState::invalidate() and marks 
the position as unknown (infinite uncertainty on the position).

What the odometry component *actually* wants to say is that the 
uncertainty is unknown, which IMO is:

   NaN NaN NaN
   NaN NaN NaN
   NaN NaN NaN

Thoughts ?

Sylvain