Absolem effort limit

[nkoenig]

Apply effort limits to the flippers.

[angelacmaio]

@peci1 For your awareness, the effort limits are changed here to match the statement in specifications.md that the robot can support itself with the flippers, but cannot use them to lift itself on all four flippers.

[peci1]

@nkoenig Have you found another solution to publish the static transforms removed in 692ce56 ? Without them, the URDF model would not work.

Also, joints left_track_j and right_track_j are so far always at zero position, but I plan to add support for the lockable differential which would allow them to move. I'd rather keep these values in the joint states...